Well-defined Conductance Research Articles

Single Dynamic Covalent Bond Tailored Responsive Molecular Junctions.

Responsive molecular devices are one of the core units for molecular electronics, and dynamic covalent bonds (DCBs) provide the opportunity for the fabrication of responsive molecular devices. Herein we employ a single dynamic acyl hydrazone bond to fabricate tailored molecular devices using the scanning tunneling microscopy break-junction technique (STM-BJ) and the eutectic Ga-In technique (EGaIn). We found that the single-DCB-tailored molecular devices exhibited acid-base and/or photo-thermal response with three well-defined molecular conductance states. The reversible switching has the ON/OFF ratio of ≈10 between each state for single-molecule junctions and ≈3 for the SAMs-based molecular junctions. Combined with the density functional theory calculations, we revealed that the multiple conductance states of these molecular junctions originate from the dynamic acyl hydrazone bond exchange and C=N isomerization. Our work opens the avenue towards the design of tailored single-molecule electrical devices by implanting dynamic covalent bonds in molecular architectures.

Fabry-Pérot resonant vortices and magnetoconductance in topological insulator constrictions with magnetic barriers

The edge states of two-dimensional time-reversal topological insulators support a perfect helical conductance on wide ribbons due to the absence of backscattering. Here, we study the changes in the transport properties of topological insulator nanoribbons by introducing a constriction along the ribbon. This setup allows the edge states to hybridize, leading to reflections at the ends of the constriction. We find that the electronic states running along one edge can be reflected back along the opposite edge multiple times, giving rise to Fabry-P\'erot resonant vortices within the constriction with well-defined conductance peaks. We show that magnetic barriers allow one to manipulate these peaks and obtain significant changes in the system spin-resolved magnetoconductance.

Gate-defined quantum point contact in an InSb two-dimensional electron gas

We investigate an electrostatically defined quantum point contact (QPC) in a high-mobility InSb two-dimensional electron gas. Well-defined conductance plateaus are observed, and the subband structure of the QPC is extracted from finite-bias measurements. The Zeeman splitting is measured in both in-plane and out-of-plane magnetic fields. We find an in-plane g factor |g∥*|≈40. The out-of-plane g factor is measured to be |g⊥*|≈50, which is close to the g factor in the bulk.Received 7 November 2020Revised 16 February 2021Accepted 11 March 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.023042Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectrical conductivityQuantum transportSpin-orbit couplingPhysical SystemsQuantum wellsTwo-dimensional electron systemTechniquesCrystal growthEpitaxyEtchingEvaporationHall barLiquid helium coolingTransport techniquesk dot p methodCondensed Matter & Materials Physics

Robust quantum point contact operation of narrow graphene constrictions patterned by AFM cleavage lithography

Detecting conductance quantization in graphene nanostructures turned out more challenging than expected. The observation of well-defined conductance plateaus through graphene nanoconstrictions so far has only been accessible in the highest quality suspended or h-BN encapsulated devices. However, reaching low conductance quanta in zero magnetic field, is a delicate task even with such ultra-high mobility devices. Here, we demonstrate a simple AFM-based nanopatterning technique for defining graphene constrictions with high precision (down to 10 nm width) and reduced edge-roughness (+/−1 nm). The patterning process is based on the in-plane mechanical cleavage of graphene by the AFM tip, along its high symmetry crystallographic directions. As-defined, narrow graphene constrictions with improved edge quality enable an unprecedentedly robust QPC operation, allowing the observation of conductance quantization even on standard SiO2/Si substrates, down to low conductance quanta. Conductance plateaus, were observed at n × e2/h, evenly spaced by 2 × e2/h (corresponding to n = 3, 5, 7, 9, 11) in the absence of an external magnetic field, while spaced by e2/h (n = 1, 2, 3, 4, 5, 6) in 8 T magnetic field.

Reliably Probing the Conductance of a Molecule in a Cavity via van der Waals Contacts

To address a long-standing issue of building molecular electrical circuits heavily relying on anchoring chemistry, we propose an alternative method to immobilize anchor-free molecules in a molecular cavity between metal electrodes. In such a scheme, well-defined conductance distribution of anchor-free molecules was obtained by means of a scanning tunneling microscopy break-junction technique. Density functional theory calculations suggest that effective electronic coupling at the molecule-electrode interface can be achieved through well-defined van der Waals (vdW) interactions when the molecule is confined in a cavity with a defined geometry. This work offers a new paradigm to achieve reliable conductance measurements of molecules via vdW interaction without metal-binding groups.

Mechanisms of jump to contact and conductance plateau formation in copper atomic junctions in vacuum and aqueous environments

The interplay between groups of water molecules and single-atom contacts, as reflected in the electrical conductances and mechanical forces of copper atomic junctions, is explored by means of first-principles theory and semi-empirical calculations. We study the influence of the atomic geometries of copper electrodes with pyramidal and non-crystalline structures in the presence and absence of water on the conductance profiles as the electrodes approach each other. It is shown that the atomic arrangements of nano-contacts have crucial effects on the formation of plateaus and the conductance values. Groups of hydrogen bonded water molecules bridge the junction electrodes before a direct Cu-Cu contact between the electrodes is made. However, the bridging of the two copper electrodes by a single H$_{2}$O molecule only occurs in the junctions with pyramidal electrodes. Our findings reveal that the presence of H$_{2}$O molecules modifies strongly the conductance profile of these junctions. In the absence of water molecules, the pyramidal junctions exhibit continuous transitions between integer conductance plateaus, while in the presence of H$_{2}$O molecules, these junctions show abrupt jump to contact behavior and no well-defined conductance plateaus. By contrast, in the absence of H$_{2}$O molecules, the non-crystalline junctions display jump to contact behavior and no well-defined plateaus, while in the presence of H$_{2}$O molecules they exhibit a jump to contact and abrupt transitions between fractional and integer plateaus.

Robust atomic-structure of the 6 × 2 reconstruction surface of Ge(110) protected by the electronically transparent graphene monolayer.

Wafer-scale growth of the unidirectional graphene monolayer on Ge surfaces has rejuvenated the intense study of the surfaces and interfaces of semiconductors underneath graphene. Recently, it was reported that the Ge atoms in the Ge(110) surface beneath a graphene monolayer underwent a rearrangement and formed an ordered (6 × 2) reconstruction. However, a plausible atomic model related to this (6 × 2) reconstruction is still lacking. Here, by using scanning tunnelling microscopy/spectroscopy (STM/S) and density functional theory (DFT) calculations, we deeply investigated the structural and electronic properties of the Ge(110) (6 × 2) surface encapsulated by a graphene monolayer. The (6 × 2) surface reconstruction was confirmed for the post-annealing-graphene-covered Ge(110) surface via STM, and was found to be quite air-stable, owing to the protection of the graphene monolayer against surface oxidation. Our study disclosed that the topographic features of the topmost graphene monolayer and the Ge(110) surface could be selectively imaged by utilizing suitable scanning biases. According to the STM results and DFT calculations, a rational ball-and-stick model of the (6 × 2) reconstruction was successfully provided, in which an elemental building block comprising two Ge triangles and two isolated Ge atoms adsorbed on the unreconstructed ideal Ge(110) surface. Local density of states of the graphene/Ge surface was explored via scanning tunneling spectroscopy (STS), presenting four well-defined differential conductance (dI/dV) peaks, protruding at energies of 0.2, 0.4, 0.6 and 0.8 eV, respectively. The four peaks predominantly originated from the surface states of the reconstructing adatoms and were well reproduced by our theoretical simulation. This result means that the Ge surface is very robust after being encapsulated by the epitaxial graphene, which could be advantageous for directly fabricating graphene/Ge-hybrid high-speed electronics and optoelectronics based on conventional microelectronics technology.

Targeting specific membranes with an azide derivative of the pore-forming peptide ceratotoxin A

Pore-forming antimicrobial peptides (AMPs) are attracting interest as cytolytic antibiotics and drug delivery agents with potential use for targeting cancer cells or multidrug-resistant pathogens. Ceratotoxin A (CtxA) is an insect-derived cytolytic AMP with 36 amino acids that is thought to protect the eggs of the medfly Ceratitis capitata against pathogens. Single channel recordings using planar lipid bilayers have shown that CtxA forms pores with well-defined conductance states resembling those of alamethicin; it also forms one of the largest pores among the group of ceratotoxins. In this work, we modified CtxA at its N-terminus with an azide group and investigated its pore-forming characteristics in planar lipid bilayer experiments. We demonstrate the possibility to target specific lipids by carrying out click reactions in-situ on lipid membranes that display a dibenzocyclooctyne (DBCO) moiety on their head group. As a result of covalent linkage of the peptides to the bilayer, pore-formation occurs at 10-fold reduced peptide concentration and with a reduced dependence on the transmembrane voltage compared to unlinked CtxA-azide peptides or native CtxA peptides.

Quantized Conductance in Topological Insulators Revealed by the Shockley-Ramo Theorem.

Crystals with symmetry-protected topological order, such as topological insulators, promise coherent spin and charge transport phenomena even in the presence of disorder at room temperature. We demonstrate how to image and read out the local conductance of helical surface modes in the prototypical topological insulators Bi_{2}Se_{3} and BiSbTe_{3}. We apply the so-called Shockley-Ramo theorem to design an optoelectronic probe circuit for the gapless surface states, and we find a well-defined conductance quantization at 1e^{2}/h within the experimental error without any external magnetic field. The unprecedented response is a clear signature of local spin-polarized transport, and it can be switched on and off via an electrostatic field effect. The macroscopic, global readout scheme is based on an electrostatic coupling from the local excitation spot to the readout electrodes, and it does not require coherent transport between electrodes, in contrast to the conventional Landauer-Büttiker description. It provides a generalizable platform for studying further nontrivial gapless systems such as Weyl semimetals and quantum spin-Hall insulators.

Fabry-Pérot interference in a triple-gated quantum point contact

We demonstrated that a triple-gated GaAs quantum point contact, which has an additional surface gate between a pair of split gates to strengthen the lateral confinement, produces the well-defined quantized conductance and Fabry-Pérot-type (FP-type) oscillations on it even using a relatively low mobility wafer. A one-dimensional phenomenological model potential was developed to explain the oscillatory behavior. By combining the model calculations and dc bias spectroscopy, we obtained a detailed information about the energy scales of the oscillatory structures. The relationships between the FP-type oscillations and the anomaly below the first plateau will be addressed.

Biomimetic Approach for Ion Channels Based on Surfactant Encapsulated Spherical Porous Metal-Oxide Capsules.

Distinguished hybrid clusters with hydrophilic and hydrophobic interiors embedded within cationic surfactant shells are spontaneously inserted into lipid bilayers, showing well-defined ionic conductance behaviors. The transport via the narrow pore gates acting as selectivity filters is controlled by the dehydration energy of the cations.

Highly conductive single naphthalene and anthracene molecular junction with well-defined conductance

We performed electronic investigation on single acene molecular junctions bridging Au-electrodes in ultra-high vacuum conditions using mechanically controllable break junction technique. While the molecular junctions displayed various conductance values at 100 K, they exhibited well-defined high conductance values (∼0.3 G0) at 300 K, which is close to that of metal atomic contact. Direct π-binding of the molecules to the Au-electrodes leads to the high conductivities at the metal-molecule interface. At the elevated temperature, single molecular junctions trapped in local metastable structures can be fallen into energetically preferential more stable state and thus we fabricated structurally well-defined molecular junctions.

Strain sensors based on chromium nanoparticle arrays

High sensitivity strain sensors have been fabricated by depositing chromium (Cr) nanoparticles between silver interdigital electrodes coated on the substrate of a flexible plastic material like polyethylene terephthalate. A gas phase cluster beam deposition system was used for the preparation of the nanoparticle arrays with a well-defined conductance. We demonstrate that the conductance of the nanoparticle arrays is dominated by the quantum transport between the closely spaced nanoparticles. Compared to the traditional metallic foil or semiconductor strain sensors, this kind of strain sensor demonstrates both a higher gauge factor, which can be as large as the order of 100, and a wider dynamic range, with a workable maximum applied strain beyond 3%.

Carbon tips as electrodes for single-molecule junctions

We study electron transport through single-molecule junctions formed by an octanethiol molecule bonded with the thiol anchoring group to a gold electrode and the opposing methyl endgroup to a carbon tip. Using the scanning tunneling microscope based break junction technique, we measure the electrical conductance of such molecular junctions. We observe the presence of well-defined conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.

Gating of Nanopores: Modeling and Implementation of Logic Gates

Nanoporous and nanofluidic structures can be coated with metal and insulating layers deposited on the pore surface: when an electrolyte solution is in contact with the internal insulating layer, well-defined ionic conductance levels could be tuned by applying a gate potential to the external metallic layer. We study theoretically the dependence of the effective gate potential at the insulating layer/solution interface with the applied gate potential at the metallic surface as well as the change of the nanopore conductance with the gate potential for different electrolyte solution concentrations and nanopore radii. We solve the Poisson−Boltzmann equation to obtain the electrical potential distribution in the two regions of the pore cross-section, the insulating layer, and the inner pore solution. The model provides estimations of the effective nanopore surface charge density that could be achieved by gating the nanopore (this charge determines the nanopore selectivity in practical cases). As an application...

Conformational and conductance fluctuations in a single-molecule junction: Multiscale computational study

The key difficulty of interpreting single-molecule junction experiments arises from the uncertainties in the molecule-metal contact configurations. As an initial step toward theoretically resolving the problem, we apply a multiscale computational approach that automates force-field (FF) molecular-dynamics (MD) simulations and density-functional theory (DFT) and matrix Green's function calculations to study the correlation between the conformational and conductance fluctuations in an ideal hexanedithiolate (C6DT) single-molecule junction model. From the 300 K MD simulations of the junction model with the molecule-metal contacts modeled by the bonding of the sulfur linker atoms to the flat Au(111) surfaces, we observe noticeable movements of the S atoms that hop between hollow sites, and confirm that the potential surface derived from (Becke-3-Lee-Yang-Parr-DFT-derived) FF is fairly consistent with that obtained with (Perdew-Burke-Ernzerhof) DFT. The corresponding conductance histogram results in a single well-defined conductance peak irrespective of the C6DT mobility, so we conclude that while the multiple conductance peaks reported in several experiments cannot be explained with the considered ideal S-Au binding geometry, it can serve as a reference for more realistic molecule-metal contact models. Because the energetically preferable hollow sites correspond to the low-value side of the conductance distribution, we find that thermal fluctuations result in a slightly increased C6DT peak conductance value compared with that from the zero-temperature energy-minimized structure and that the conductance histogram can be better fit on a logarithmic scale.

Rigid adamantane tripod linkage for well-defined conductance of a single-molecule junction

We present a systematic analysis of molecular level alignments and electron transport characteristics based on the non-equilibrium Green's function (NEGF) approach combined with density functional theory (DFT) for phenyl dithiol (PDT) derivatives with two different linkages, simple thiol (ST) and tripod-shaped adamantane trithiol (ATT). The substantial adamantane-metal bonds afford a rigid contact structure and well-defined conductance of a single-molecule junction irrespective of interfacial phenomena, which accordingly allows us to utilize the intrinsic nature of the molecule for designing molecular devices with prescribed transport characteristics. We suggest a feasible application of the ATT linkage embedded single-molecule junction to a molecular transistor based on the practical feature of the ATT linkage, which is the unsusceptible behavior of transmission under the applied bias voltage. The present result may serve as an important reference point for designing molecular devices with prescribed transport properties.

Structural Properties of Pore-Forming Oligomers of α-Synuclein

Soluble oligomers are potent toxins in many neurodegenerative diseases, but little is known about the structure of soluble oligomers and their structure-toxicity relationship. Here we prepared on-pathway oligomers of the 140-residue protein alpha-synuclein, a key player in Parkinson's disease, at concentrations an order of magnitude higher than previously possible. The oligomers form ion channels with well-defined conductance states in a variety of membranes, and their beta-structure differs from that of amyloid fibrils of alpha-synuclein.

EtpB Is a Pore-Forming Outer Membrane Protein Showing TpsB Protein Features Involved in the Two-Partner Secretion System

Attachment to host tissues is a critical step in the pathogenesis of most bacterial infections. Enterotoxigenic Escherichia coli (ETEC) remains one of the principal causes of infectious diarrhea in humans. The recent identification of additional ETEC surface molecules suggests that new targets may be exploited in vaccine development. The EtpA protein identified in ETEC H10407 is a large glycosylated adhesin secreted via the two-partner secretion system. EtpA requires its putative partner EtpB for translocation across the outer membrane (OM). We investigated the biochemical and electrophysiological properties of purified EtpB. We showed that EtpB is 65-kDa heat-modifiable protein localized to the OM. Electrophysiological experiments indicated that EtpB is able to form pores in planar lipid bilayer membranes with an asymmetric current, suggesting its functional asymmetry. The pore of EtpB frequently assumes an opened conformation and fluctuates between three well-defined conductance states. In silico analysis of the EtpB amino acid sequence and molecular modeling suggest that EtpB is similar to the well-known TpsB protein FhaC from Bordetella pertussis and has a C-terminal transmembrane beta-barrel domain that is occluded by an N-terminal alpha-helix, an extracellular loop, and two periplasmic polypeptide-transport-associated (POTRA) domains. Together, these data confirm that EtpB is a pore-forming protein mainly folded into a beta-barrel conformation and indicate that EtpB presents typical features of the OM TpsB proteins.

Fabrication and conductance characterization of single C 60 molecular junction in solutions

The conductance and stretching length of the single C 60 molecule bridging two Au electrodes (C 60 molecular junction) were studied in air, mesitylene and 0.1 M HClO 4 solution. In air, the C 60 molecular junction did not show any well-defined conductance value. In either mesitylene or HClO 4 solution, the systems showed a conductance value of 0.01 G 0. The stretching length of the single C 60 molecular junction was determined to be 0.01 nm. The values of the conductance and stretching length in solution were smaller than those obtained in ultra high vacuum, indicating the weak C 60–Au interaction in solution. Considering the particular shape of the C 60 molecule and the junction formation process in the present system, a sliding model was proposed to interpret the observation.