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The orientation and rearrangement of water on a gold electrode significantly influences its physicochemical heterogeneous performance. Despite numerous experimental and theoretical studies aimed at uncovering the structural characteristics of interfacial water, the orientational behavior resulting from electrode-induced rearrangements remains a subject of ongoing debate. Here, we employed molecular dynamics simulations to investigate the adaptive structure and dynamics properties of interfacial water on Au(111) and Au(100) surfaces by considering a polarizable model for Au atoms in comparison with the non-polarizable model. Compared to the nonpolarizable systems, the polarization effect can enhance the interaction between water molecules and the gold surface. Unexpectedly, the rotational dynamics directly associated with the orientational behavior of water adjacent to the gold surface is accelerated, thereby reducing the hydrogen bond lifetime. The underlying mechanism for this anomalous phenomenon originates from the polarization effect, which induces the attraction of the positive hydrogen atoms to the surface by the negative image charge. This leads to a change in orientation that disrupts the hydrogen bonds in the first water layer and subsequently accelerates reorientation dynamics of water molecules adjacent to the gold surface. These results shed light on the intricate interplay between polarization effects and water molecule dynamics on metal surfaces, establishing the foundation for the rational regulation of the orientation of interfacial water. [ABSTRACT FROM AUTHOR]

By means of the first-principles method, the bias effect on the magnetism of polyacene (n-acene) connected to gold electrodes is investigated. A magnetic to nonmagnetic transition for the polyacene (n > 6) is observed when the bias exceeds a critical value. The mechanism is explored as the bias-induced variation of electronic localization, which leads to the exchange of dominant mechanism for molecular magnetism from Columbic interaction between electrons to electron hopping rate. A significant enhancement of the differential conductance and suppression of current spin polarization for the molecular device are also obtained accompanied by the transition of molecular magnetism. This work proposes a feasible way to manipulate the magnetism of polyacene via electric method and reveals the relation between molecular magnetism and its conductance. [ABSTRACT FROM AUTHOR]

Single molecule junctions based on selected 4,4′-biphenyldithiol and 4,4′-dicyanobiphenyl derivatives bonded to gold electrodes are analyzed from a dynamical point of view. A fully quantum mechanical description of the internal rotation of the biphenyl moiety is carried out in terms of the nuclear wavepacket dynamics obtained by the solution of the time-dependent Schrödinger equation expressed in terms of the torsion angle between the phenyl rings. The required potential energy surfaces are computed using ab initio electronic structure methods. The nature and positions of the substituents on the phenyl rings determine the features of the potential energy surfaces. The effect of the initial conditions on the time propagation of the nuclear wavepackets and, as a consequence, on the evolution of the conformational distribution is also analyzed. In addition, the conductances at zero bias for the nanojunctions were computed for different conformations of the biphenyl fragments. Weighted by the wavepacket amplitudes, non-stationary conductance expectation values, and time-averaged torsion angles and conductances for the entire simulation are obtained. The consequences of using the time-averaged values to perform a linear regression between the conductance and the square of the cosine of the dihedral angle between the phenyl rings are analyzed and compared to the usual static approach based only on the information for equilibrium geometries. The study of the time dependent conformational variations of the biphenyl moieties in the nanojunctions allows for a better understanding of the quantum chemical phenomena that affect their transport properties. [ABSTRACT FROM AUTHOR]

Understanding the electric double layer (EDL) of the metal electrode–electrolyte interface is essential to electrochemistry and relevant disciplines. In this study, potential-dependent electrode Sum Frequency Generation (SFG) intensities of polycrystalline gold electrodes in HClO4 and H2SO4 electrolytes were thoroughly analyzed. The potential of zero charges (PZC) of the electrodes was −0.06 and 0.38 V in HClO4 and H2SO4, respectively, determined from differential capacity curves. Without specific adsorption, the total SFG intensity was dominated by the contribution from the Au surface and increased similar to that of the visible (VIS) wavelength scanning, which pushed the SFG process closer to the double resonant condition in HClO4. However, the EDL contributed about 30% SFG signal with specific adsorption in H2SO4. Below PZC, the total SFG intensity was dominated by the Au surface contribution and increased with potential at a similar slope in these two electrolytes. Around PZC, as the EDL structure became less ordered and the electric field changed direction, there would be no EDL SFG contribution. Above PZC, the total SFG intensity increased much more rapidly with potential in H2SO4 than in HClO4, which suggested that the EDL SFG contribution kept increasing with more specific adsorbed surface ions from H2SO4. [ABSTRACT FROM AUTHOR]

We demonstrate single molecule conductance as a sensitive and atomically precise probe of binding configurations of adenine and its biologically relevant variants on gold. By combining experimental measurements and density functional theory (DFT) calculations of single molecule–metal junction structures in aqueous conditions, we determine for the first time that robust binding of adenine occurs in neutral or basic pH when the molecule is deprotonated at the imidazole moiety. The molecule binds through the donation of the electron lone pairs from the imidazole nitrogen atoms, N7 and N9, to the gold electrodes. In addition, the pyrimidine ring nitrogen, N3, can bind concurrently and strengthen the overall metal–molecule interaction. The amine does not participate in binding to gold in contrast to most other amine-terminated molecular wires due to the planar geometry of the nucleobase. DFT calculations reveal the importance of interface charge transfer in stabilizing the experimentally observed binding configurations. We demonstrate that biologically relevant variants of adenine, 6-methyladenine and 2′-deoxyadenosine, have distinct conductance signatures. These results lay the foundation for biosensing on gold using single molecule conductance readout. [ABSTRACT FROM AUTHOR]

Maintaining stability of single-molecular junctions (SMJs) in the presence of current flow is a prerequisite for their potential device applications. However, theoretical understanding of nonequilibrium heat transport in current-carrying SMJs is a challenging problem due to the different kinds of nonlinear interactions involved, including electron–vibration and anharmonic vibrational coupling. Here, we overcome this challenge by accelerating Langevin-type current-induced molecular dynamics using machine-learning potential derived from density functional theory. We show that SMJs with graphene electrodes generate an order of magnitude less heating than those with gold electrodes. This is rooted in the better phonon spectral overlap of graphene with molecular vibrations, rendering harmonic phonon heat transport being dominant. In contrast, in a spectrally mismatched junction with gold electrodes, anharmonic coupling becomes important to transport heat away from the molecule to surrounding electrodes. Our work paves the way for studying current-induced heat transport and energy redistribution in realistic SMJs. [ABSTRACT FROM AUTHOR]

Understanding the response of the surface of metallic solids to external electric field sources is crucial to characterize electrode–electrolyte interfaces. Continuum electrostatics offer a simple description of the induced charge density at the electrode surface. However, such a simple description does not take into account features related to the atomic structure of the solid and to the molecular nature of the solvent and of the dissolved ions. In order to illustrate such effects and assess the ability of continuum electrostatics to describe the induced charge distribution, we investigate the behavior of a gold electrode interacting with sodium or chloride ions fixed at various positions, in a vacuum or in water, using all-atom constant-potential classical molecular dynamics simulations. Our analysis highlights important similarities between the two approaches, especially under vacuum conditions and when the ion is sufficiently far from the surface, as well as some limitations of the continuum description, namely, neglecting the charges induced by the adsorbed solvent molecules and the screening effect of the solvent when the ion is close to the surface. While the detailed features of the charge distribution are system-specific, we expect some of our generic conclusions on the induced charge density to hold for other ions, solvents, and electrode surfaces. Beyond this particular case, the present study also illustrates the relevance of such molecular simulations to serve as a reference for the design of improved implicit solvent models of electrode–electrolyte interfaces. [ABSTRACT FROM AUTHOR]

Electrochemistry experiments have established that the capacitance of electrode–electrolyte interfaces is much larger for good metals, such as gold and platinum, than for carbon-based materials. Despite the development of elaborate electrode interaction potentials, to date molecular dynamics simulations are not able to capture this effect. Here, we show that changing the width of the Gaussian charge distribution used to represent the atomic charges in gold is an effective way to tune its metallicity. Larger Gaussian widths lead to a capacitance of aqueous solutions (pure water and 1 M NaCl) in good agreement with recent ab initio molecular dynamics results. For pure water, the increase in the capacitance is not accompanied by structural changes, while in the presence of salt, the Na+ cations tend to adsorb significantly on the surface. For a strongly metallic gold electrode, these ions can even form inner sphere complexes on hollow sites of the surface. [ABSTRACT FROM AUTHOR]

We report the use of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a transducing material in the fabrication of mechanical-to-electrical conversion devices based on the flexoelectric-like response of this polymer. Devices are made in a cantilever-based three-layer stainless-steel/PEDOT:PSS/top metal electrode configuration to assess the effective transverse flexoelectric coefficient μ 12 ′ . We investigated the influence of the nature of the top electrode in the flexoelectric response comparing samples with gold and aluminum top electrodes and demonstrated the huge impact of adding a small fraction of a second dopant such as xylitol to the PEDOT:PSS polymer blend and the benefits of a post-treatment of the polymer film with ethanol and methanol on the flexoelectric coefficient. The combination of xylitol addition and the rinsing of the polymer films with ethanol and methanol, along with the use of gold as a top electrode, led to a significant improvement of μ 12 ′ to ca. 24 μC m−1, which is in the range of those reported for high permittivity oxide materials. These findings support the use of conjugated polymers as an alternative to inorganic materials in flexoelectric-based applications, where large flexibility is required. [ABSTRACT FROM AUTHOR]

Nanowires have emerged as promising one-dimensional materials with which to construct various nanocircuits and nanosensors. However, measuring the electrical properties of individual nanowires directly remains challenging because of their small size, thereby hindering the comprehensive understanding of nanowire-based device performance. A crucial factor in achieving reliable electrical characterization is establishing well-determined contact conditions between the nanowire sample and the electrodes, which becomes particularly difficult for soft nanowires. Introduced here is a novel technique for measuring the conductivity of an individual nanowire with the aid of automated nanomanipulation using an atomic force microscope. In this method, two nanowire segments cut from the same silver nanowire are positioned onto a pair of gold electrodes, serving as flexible nanoprobes to establish controllable contact with the sample. By changing the contact points along the nanowire sample, conductivity measurements can be performed on different regions, thereby eliminating the influence of contact resistance by analyzing multiple current–voltage curves. Using this approach, the resistivity of a 100-nm-diameter silver nanowire is determined to be 3.49 × 10−8 Ω m. ARTICLE HIGHLIGHTS: HIGHLIGHTS • Two flexible nanoprobes in the form of silver nanowires were fabricated using a series of complex AFM manipulation strategies including manipulating individual nanowires via translation, rotation, and cutting. • In situ measurements of the electrical conductivity of an individual nanowire were conducted at multiple points using the flexible nanoprobes, thereby effectively mitigating the influence of contact resistance and enabling determination of the nanowire's resistivity. [ABSTRACT FROM AUTHOR]

Organic color-centers (OCCs) on single-wall carbon nanotubes are quantum defects that demonstrate intriguing near-infrared emission properties with potential for bioimaging, chemical sensing, and quantum communication. Many of these applications will require electrical pumping rather than optical excitation to deterministically access the near-infrared emission properties of OCCs, though this has yet to be achieved. In this work, we report experimental observation of near-infrared electroluminescence from 4-nitroaryl OCCs intentionally introduced on (6,5)-single-wall carbon nanotubes that are aligned across a pair of gold electrodes. Spatially correlated photoluminescence and electroluminescence spectroscopy reveal direct evidence of the localized electroluminescence from the OCCs on the semiconducting nanotube hosts. The electroluminescence intensity displays an exponential dependence on the source–drain current, suggesting that impact excitation by unipolar carriers at the quantum defects is the origin of the observed emission. These electroluminescent quantum defects may pave the way to enable on-chip integration for potential applications of OCCs in display, sensor, and spin-based devices, as well as other quantum technologies. [ABSTRACT FROM AUTHOR]

One means for describing electron transport across single molecule tunnel junctions (MTJs) is to use density functional theory (DFT) in conjunction with a nonequilibrium Green's function formalism. This description relies on interpreting solutions to the Kohn–Sham (KS) equations used to solve the DFT problem as quasiparticle (QP) states. Many practical DFT implementations suffer from electron self-interaction errors and an inability to treat charge image potentials for molecules near metal surfaces. For MTJs, the overall effect of these errors is typically manifested as an overestimation of electronic currents. Correcting KS energies for self-interaction and image potential errors results in MTJ current–voltage characteristics in close agreement with measured currents. An alternative transport approach foregoes a QP picture and solves for a many-electron wavefunction on the MTJ subject to open system boundary conditions. It is demonstrated that this many-electron method provides similar results to the corrected QP picture for electronic current. The analysis of these two distinct approaches is related through corrections to a junction's electronic structure beyond the KS energies for the case of a benzene diamine molecule bonded between two gold electrodes. [ABSTRACT FROM AUTHOR]

The kinetics of the inner-sphere electron transfer reaction between a gold electrode and CO2 was measured as a function of the applied potential in an aqueous environment. Extraction of the electron transfer rate constant requires deconvolution of the current associated with CO2 reduction from the competing hydrogen evolution reaction and mass transport. Analysis of the inner-sphere electron transfer reaction reveals a driving force dependence of the rate constant that has similar characteristics to that of a Marcus–Hush–Levich outer-sphere electron transfer model. Consideration of simple assumptions for CO2 adsorption on the electrode surface allows for the evaluation of a CO2,ads/CO2•−ads standard potential of ∼−0.75 ± 0.05 V vs Standard Hydrogen Electrode (SHE) and a reorganization energy on the order of 0.75 ± 0.10 eV. This standard potential is considerably lower than that observed for CO2 reduction on planar metal electrodes (∼>–1.4 V vs SHE for >10 mA/cm2), thus indicating that CO2 reduction occurs at a significant overpotential and thus provides an imperative for the design of better CO2 reduction electrocatalysts. [ABSTRACT FROM AUTHOR]

In this study, the effect of aging on current transport properties and the interface band structure of the Au/Nb-doped SrTiO3 (NSTO) junction was investigated. The as-fabricated Au/NSTO junctions showed extremely small rectification and exhibited rectifying current conduction and colossal electroresistance (CER) effect after exposure to air. In contrast, the Au/NSTO junctions kept in vacuum did not exhibit such aging behavior. Interestingly, the Au/NSTO junction capped with a Pt overlayer showed small rectification even after air exposure for one week. The changes in the potential profile with increasing air exposure time were assessed by X-ray photoemission techniques. The aforementioned time-dependent behaviors originated because of oxygen diffusion through the Au electrode, and it was found that oxygen stoichiometry at the electrode interface played an important role in the CER phenomena at the metal/NSTO junctions. [ABSTRACT FROM AUTHOR]

Aiming at a detailed understanding of the interaction between an ionic liquid, O2, and electrodes in Mg-air batteries, we performed a combined differential electrochemical mass spectrometry and in situ infrared spectroscopy model study on the interaction between the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP-TFSI) and a gold film electrode in the presence and absence of O2 and Mg2+ ions in the potential range relevant for the oxygen reduction reaction (ORR) and evolution reaction. Detailed information on the dynamic exchange of adsorbed ions, on the stability/decomposition of the ionic liquid, and on the activity/selectivity/reversibility of the ORR is derived from measurements performed under potentiodynamic and potentiostatic conditions. In neat BMP-TFSI, we find the dynamics of the potential induced exchange of adsorbed ions to depend significantly on the exchange direction. In the presence of O2, the anions formed in the ORR distinctly affect the adsorption characteristics of the IL ions and the exchange dynamics. Furthermore, the ORR changes from reduction to superoxide anions at moderate potentials to reduction to peroxide anion at more negative potentials. In the additional presence of Mg2+ ions, dominant magnesium peroxide and oxide formation result in an irreversible ORR, in contrast to the requirements of an efficient re-chargeable Mg-air battery. In addition, these ions result in the increasing formation of a blocking adlayer, reducing the coverage of adsorbed IL species. [ABSTRACT FROM AUTHOR]

Bi2Sr2CaCu2O8+δ (BSCCO) single crystals are promising for high-temperature superconducting electronic devices with ultimate performance. Recently, tailoring superconductivity in ultra-thin BSCCO locally has received wide attention and interest. In this work, we focus on controlling the superconductivity of the uppermost CuO2 double layer of the BSCCO crystal. Gold electrodes are deposited onto the freshly cleaved BSCCO surface by a high-vacuum in situ evaporation technique at room temperature, and then surface Josephson junctions (SJJs) between the two outermost superconducting CuO2 double layers are fabricated as a probe to evaluate the superconductivity of the uppermost double layer. We find that the junction transition temperature Tc′ as well as its critical current density jc depend systematically and reproducibly on the deposition rate of the gold electrode. The electric properties of the SJJs, spanning the range from completely degraded to almost as good as the intrinsic Josephson junctions inside the bulk, systematically reveal the evolutionary process of superconductivity of the uppermost BSCCO layer, extending the range for practical applications. [ABSTRACT FROM AUTHOR]

In this work, we simulated the smallest fullerene molecule, C20 in a two-probe device model with gold electrodes. The gold electrodes comprised of (011) miller planes were carved to construct the novel geometry based four unique shapes, which were strung to fullerene molecules through mechanically controlled break junction techniques. The organized devices were later scrutinized using non-equilibrium Green's function based on the density functional theory to calculate their molecular orbitals, energy levels, charge transfers, and electrical parameters. After intense scrutiny, we concluded that five-edged and six-edged devices have the lowest and highest current-conductance values, which result from their electrode-dominating and electrode-subsidiary effects, respectively. However, an interesting observation was that the three-edged and four-edged electrodes functioned as semi-metallic in nature, allowing the C20 molecule to demonstrate its performance with the complementary effect of these electrodes in the electron conduction process of a two-probe device. [ABSTRACT FROM AUTHOR]

Commanding charge carrier diffusion in semiconducting channels requires the precise and realistic experimental realization of electronic energy band alignments at the interfaces and within the channels. We have demonstrated a non-contact and direct way to accurately probe the energy band bending at nanoscale spatial precision on MoS2 flakes laid on gold electrodes by mapping the surface potential landscape at non-equilibrium conditions during carrier injection. By systematically varying the charge carrier injection, the contrast gradient in surface potential profiles is studied along the MoS2 channel. Corresponding interfacial parameters, such as surface electric field (ξ), built-in potential ( Ψ b i ), and space charge density (σ), are experimentally determined. [ABSTRACT FROM AUTHOR]

We perform temperature dependent conductance measurements on sub-nanometer sized single molecules bound to gold electrodes using a scanning tunneling microscope-based break junction technique in Ultra-High Vacuum (UHV). We find a threefold increase in the conductance of amineterminated conjugated molecules when the temperature increases from 4 K to 300 K in UHV. Furthermore, the conductance measured at 300 K in UHV is consistent with solution-based measurements under ambient conditions where the transport mechanism corresponds to off-resonant electron tunneling across the molecule. Our measurements indicate that at 300 K, conductance is largely independent of pressure or solvent around the junction. In addition, our data unambiguously show that temperature can affect the tunneling conductance of single molecule-metal junctions. We show that the structure of the metal electrodes that form in these junctions varies systematically with temperature, and hypothesize that this changing structure of the interface alters electron tunneling probability and propose a mechanism to explain our findings. [ABSTRACT FROM AUTHOR]

Crown-ether molecules are well known to selectively bind alkali atoms, so by incorporating these within wires, any change in electrical conductance of the wire upon binding leads to discriminating sensing. Using a density functional theory-based approach to quantum transport, we investigate the potential sensing capabilities of single-molecule junctions formed from crown ethers attached to anthraquinone units, which are in turn attached to gold electrodes via alkyl chains. We calculate the change in electrical conductance for binding of three different alkali ions (lithium, sodium, and potassium). Depending on the nature of the ionic analyte, the conductance is enhanced by different amounts. This change in electrical conductance is due to charge transfer from the ion to molecular wire causing the molecular resonances to shift closer to the electrode Fermi energy. [ABSTRACT FROM AUTHOR]

We present a novel Faradaic reaction-free nucleic acid amplification (NAA) method for use with microscale liquid samples. Unlike previous Joule heating methods where the electrodes produce electrolysis gaseous by-products and require both the electrodes be isolated from a sample and the venting of produced electrolysis gas, our electrokinetic Nucleic Acid Amplification (E-NAAMP) method alleviates these issues using a radio frequency (RF) alternating current electric field. In this approach, a pair of microscale thin film gold electrodes are placed directly in contact with a nucleic acid reaction mixture. A high frequency (10–40 MHz) RF potential is then applied across the electrode pair to induce a local Ohmic current within the sample and drive the sample temperature to increase by Joule heating. The temperature increase is sustainable in that it can be generated for several hours of constant use without generating any pH change to the buffer or any microscopically observable gaseous electrolysis by-products. Using this RF Joule heating approach, we demonstrate successful direct thermal amplification using two popular NAA biochemical reactions: loop-mediated isothermal amplification and polymerase chain reaction. Our results demonstrate that a simple microscale electrode structure can be used for thermal regulation for NAA reactions without observable electrolytic reactions, minimal enzyme activity loss and sustained (>50 h use per device) continuous operations without electrode delamination. As such, E-NAAMP offers substantial miniaturization of the heating elements for use in microfluidic or miniaturized NAA reaction systems. [ABSTRACT FROM AUTHOR]

We have studied the electronic transport properties of armchair graphene nanoribbons (AGNRs) bridged between two metal electrodes or supported on insulating substrates in 10 nm-scale devices using the first-principles calculations. The two metal species of Ti and Au are examined as metal electrodes and are compared. The current densities through the AGNR-Ti contact are about 10 times greater than those through the AGNR-Au contact, even though the AGNR width reaches 12nm. For the insulating substrates, we have investigated the dependence of the channel length on the transport properties using models with two channel lengths of 15.1 and 9.91 nm. Regardless of the channel length, the on/off current ratio is 105 for the AGNRs on an O-terminated surface. This ratio is consistent with the recent experiments and is less by factors of 1016 for the 15.1nm channel length and 108 for the 9.91nm channel length compared to the freestanding AGNR. [ABSTRACT FROM AUTHOR]

The magnetic anisotropy and Kondo phenomena in a mechanically stretched magnetic molecular junction are investigated by combining the density functional theory (DFT) and hierarchical equations of motion (HEOM) approach. The system is comprised of a magnetic complex Co(tpy-SH)2 sandwiched between adjacent gold electrodes, which is mechanically stretched in experiments done by Parks et al. [Science 328, 1370 (2010)]. The electronic structure and mechanical property of the stretched system are investigated via the DFT calculations. The HEOM approach is then employed to characterize the Kondo resonance features, based on the Anderson impurity model parameterized from the DFT results. It is confirmed that the ground state prefers the S = 1 local spin state. The structural properties, the magnetic anisotropy, and corresponding Kondo peak splitting in the axial stretching process are systematically evaluated. The results reveal that the strong electron correlations and the local magnetic properties of the molecule magnet are very sensitive to structural distortion. This work demonstrates that the combined DFT+HEOM approach could be useful in understanding and designing mechanically controlled molecular junctions. [ABSTRACT FROM AUTHOR]

Based on the nonequilibrium Green's function formalism and density-functional theory, we investigate the onset of electrical rectification in a single C59N molecule in conjunction with gold electrodes. Our calculations reveal that rectification is dependent upon the anchoring of the Au atom on C59N; when the Au electrode is singly bonded to a C atom (labeled here as A), the system does not exhibit rectification, whereas when the electrode is connected to the C-C bridge site between two hexagonal rings (labeled here as B), transmission asymmetry is observed, where the rectification ratio reaches up to 2.62 at ±1 V depending on the N doping site relative to the anchoring site. Our analysis of the transmission mechanism shows that N doping of the B configuration causes rectification because more transmission channels are available for transmission in the B configuration than in the A configuration. [ABSTRACT FROM AUTHOR]

Advances in nanofabrication techniques have underpinned the recent growing interest in vacuum channel transistors due to their ability to showcase ballistic transport and immunity to most radiations. However, the geometry of the electrodes plays an important role in the overall performance and efficiency of the device. Several studies on the geometry of the source electrode have been carried out because of its role in enhancement of the local electric field, which triggers electron tunneling, yet the geometry of the drain electrode has been neglected. Here, we propose a new planar vacuum channel transistor with gold electrodes. Our study shows that the unique geometrical design of the device leads to high collection efficiency. Furthermore, it reveals the importance of the geometry of the drain on device performance. Emission characteristics of the device and the effect of geometrical parameters such as channel length and source geometry on its performance have also been investigated. [ABSTRACT FROM AUTHOR]

We present ab initio transport calculations for molecular junctions that include graphene as a protecting layer between a single molecule and gold electrodes. This vertical setup has recently gained significant interest in experiment for the design of particularly stable and reproducible devices. We observe that the signals from the molecule in the electronic transmission are overlayed by the signatures of the graphene sheet, thus raising the need for a reinterpretation of the transmission. On the other hand, we see that our results are stable with respect to various defects in the graphene. For weakly physiosorbed molecules, no signs of interaction with the graphene are evident, so the transport properties are determined by offresonant tunnelling between the gold leads across an extended structure that includes the molecule itself and the additional graphene layer. Compared with pure gold electrodes, calculated conductances are about one order of magnitude lower due to the increased tunnelling distance. Relative differences upon changing the end group and the length of the molecule on the other hand, are similar. [ABSTRACT FROM AUTHOR]

The electronic transport properties of benzene-porphyrin-benzene (BPB) molecules coupled to gold (Au) electrodes were investigated. By successively removing the front-end Au atoms, several BPB junctions with different molecule-electrode contact symmetries were constructed. The calculated current-voltage (I-V) curves depended strongly on the contact configurations between the BPB molecules and the Au electrodes. In particular, a significant low-voltage negative differential resis-tance effect appeared at -0.3 V in the junctions with pyramidal electrodes on both sides. Along with the breaking of this tip-contact symmetry, the low-bias negative differential resistance ef-fect gradually disappeared. This tip-contact may be ideal for use in the design of future molec-ular devices because of its similarity with experimental processes. [ABSTRACT FROM AUTHOR]

The atomic structure and the electron transfer properties of hydrogen bonds formed between two carboxylated alkanethiol molecules connected to gold electrodes are investigated by employing the non-equilibrium Green's function formalism combined with density functional theory. Three types of molecular junctions are constructed, in which one carboxyl alkanethiol molecule contains two methylene, -CH2, groups and the other one is composed of one, two, or three -CH2 groups. Our calculations show that, similarly to the cases of isolated carboxylic acid dimers, in these molecular junctions the two carboxyl, -COOH, groups form two H-bonds resulting in a cyclic structure. When self-interaction corrections are explicitly considered, the calculated transmission coefficients of these three H-bonded molecular junctions at the Fermi level are in good agreement with the experimen-tal values. The analysis of the projected density of states confirms that the covalent Au-S bonds localized at the molecule-electrode interfaces and the electronic coupling between -COOH and S dominate the low-bias junction conductance. Following the increase of the number of the -CH2 groups, the coupling between -COOH and S decreases deeply. As a result, the junction conductance decays rapidly as the length of the H-bonded molecules increases. These findings not only provide an explanation to the observed distance dependence of the electron transfer properties of H-bonds, but also help the design of molecular devices constructed through H-bonds. [ABSTRACT FROM AUTHOR]

Based on a quantum chemical valence formalism that allows the rigorous construction of bestlocalized molecular orbitals on specific parts of an extended system, we examined the separability of individual components of model systems relevant to the description of electron transport in molecular devices. We started by examining how to construct the maximally localized electronic density at the tip of a realistic model of a gold electrode. By varying the number of gold atoms included in the local region where to project the total electronic density, we quantitatively assess how many molecular orbitals are entirely localized in that region. We then considered a 1,4-benzene-di-thiol molecule connected to two model gold electrodes and examined how to localize the electronic density of the total system in the extended molecule, a fractional entity comprising the organic molecule plus an increasing number of the closest metal atoms. We were able to identify in a rigorous manner the existence of three physically different electronic populations, each one corresponding to a distinct set of molecular orbitals. First, there are those entirely localized in the extended molecule, then there is a second group of those completely distributed in the gold atoms external to that region, and, finally, there are those delocalized over the entire system. This latter group can be associated to the shared electronic population between the extended molecule and the rest of the system. We suggest that the treatment here presented could be useful in the theoretical analysis of the electronic transport in nanodevices whenever the use of localized molecular states are required by the physics of the specific problem, such as in cases of weak coupling and super-exchange limits. [ABSTRACT FROM AUTHOR]

Various domain structures, including wave-like domains, mixed needle-like and laminar domains, typical embedded 90° and 180° domains, have been observed in unpoled rhombohedral, monoclinic, and tetragonal Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) crystals by polarizing light microscope; while in poled tetragonal crystals, the parallel 180° domains were reversed and only vertical 90° domain walls were observed. For 0.24PIN-0.42PMN-0.34PT crystals with morphotropic phase boundary composition, the domain wall motion was in-situ observed as a function of applied electric field along crystallographic [100] direction. With increasing the electric field from 0 to 12 kV/cm, the rhombohedral (R) domains were found to change to monoclinic (M) domains and then to tetragonal (T) domains. The electric field-induced phase transition was also confirmed by X-ray diffraction and the temperature-dependent dielectric behavior. [ABSTRACT FROM AUTHOR]

With progressively improving charge carrier mobility in organic semiconductors and miniaturization of organic field-effect transistors (OFETs), low contact resistances become ever more important. To study the capabilities of metal electrodes in OFETs and to explore the transition from contact-limited to channel-dominated transistor operation, we used flip-crystal FETs with gold electrodes having different contact resistances Rc to high-quality rubrene crystals. 4-terminal transfer and output measurements reveal that Rc decreases from 105-106 Ω cm for 15 min air exposure to 3×10³ Ωcm for at least 5 h air exposure of the gold electrodes before the flip-crystal FET is assembled. We conclude the reduction of Rc to be caused by a growing contamination layer on the gold electrodes that weakens the electrostatic coupling between rubrene crystal and gold electrode, and lowers the Schottky contact diode parameter V0. In channel-dominated (low Rc) FETs, the mobility is in the range of 10-17 cm²/(Vs); in contrast, in contact-limited (high Rc) FETs, the apparent mobility decreases significantly with increasing contact resistance. The apparent μ - Rc dependence is not intrinsic, but rather the result of incorrect assumptions of the potential and the charge carrier density in the channel region. Thus, the development of high-mobility organic semiconductors requires further efforts to improve contacts beyond traditional metal electrodes. [ABSTRACT FROM AUTHOR]

In this experimental study, a portable biosensor was developed to detect β-human chorionic gonadotropin (β-hCG), which is extensively used in pregnancy tests and serves as a biomarker for ectopic pregnancy. The sensor used is an electric-double-layer field-effect transistor biosensor with the extended-gate design. Bias voltage is applied on the sensor to measure the resulting drain current signals. Gold electrode surface is functionally activated with an anti-β-hCG antibody to capture β-hCG protein. Fluorescence imaging technique is utilized to confirm the surface functionalization. The biosensor demonstrates a dynamically wide range of molecules as detection targets at very low sample concentrations, which shows the potential to detect ectopic pregnancy in very early stages and easily keep track of its periodic changes. It can be produced en masse and does not use additional labels/reagents or pre-processing techniques for the sample. This biosensor can significantly reduce the manufacturing costs and is comparable with the currently available commercial ß-hCG assays. It is suitable for early diagnosis of ectopic pregnancy with low cost and easy operation at home with urine samples. [ABSTRACT FROM AUTHOR]

We design isolated molecular nanowires composed of thiophene oligomers sandwiched between two one-dimensional gold electrodes. Electronic transport through the molecular junctions with two interface geometries is studied by performing the first principles calculations based on density functional theory and nonequilibrium Green's function. The current-voltage (I-V) curves of the molecular wires display an unexpected negative differential resistance and rectifying behaviors along with the oscillation effects, different from other theoretical and experimental studies about the analogous thiophene devices. The significant difference is attributed to the design of the one-dimensional gold electrodes with large enough vacuum layer in transverse direction in order to suppress the interaction between wires. Such transport behaviors indicate that the thiophene molecular device would be an important candidate in future molecular electronics. [ABSTRACT FROM AUTHOR]

As electronics devices scale to sub-10 nm lengths, the distinction between 'device' and 'electrodes' becomes blurred. Here, we study a simple model of a molecular tunnel junction, consisting of an atomic gold chain partitioned into left and right electrodes, and a central 'molecule.' Using a complex absorbing potential, we are able to reproduce the single-particle energy levels of the device region including a description of the effects of the semi-infinite electrodes. We then use the method of configuration interaction to explore the effect of correlations on the system's quasiparticle peaks. We find that when excitations on the leads are excluded, the device's highest occupied molecular orbital and lowest unoccupied molecular orbital quasiparticle states when including correlation are bracketed by their respective values in the Hartree-Fock (Koopmans) and ΔSCF approximations. In contrast, when excitations on the leads are included, the bracketing property no longer holds, and both the positions and the lifetimes of the quasiparticle levels change considerably, indicating that the combined effect of coupling and correlation is to alter the quasiparticle spectrum significantly relative to an isolated molecule. [ABSTRACT FROM AUTHOR]

Tumor-treating fields (TTFields) are alternating electrical fields of intermediate frequency and low intensity that can slow or inhibit tumor growth by disrupting mitosis division of cancerous cells through cell cycle proteins. In this work, for the first time, an in-house fabricated cyclo-olefin polymer made microfluidic bioreactors are integrated with Cr/Au interdigitated electrodes to test TTFields on yeast cells with fluorescent protein:Nop56 gene. A small gap between electrodes (50 μm) allows small voltages (<150 mV) to be applied on the cells; hence, uninsulated gold electrodes are used in the non-faradaic region without causing any electrochemical reaction at the electrode-medium interface. Electrochemical modeling as well as impedance characterization and analysis of the electrodes are done using four different cell nutrient media. The experiments with yeast cells are done with 150 mV, 150 kHz and 30 mV, 200 kHz sinusoidal signals to generate electrical field magnitudes of 6.58 V/cm and 1.33 V/cm, respectively. In the high electrical field experiment, the cells go through electroporation. In the experiment with the low electrical field magnitude for TTFields, the cells have prolonged mitosis from typical 80–90 min to 200–300 min. Our results confirm the validity of the electrochemical model and the importance of applying a correct magnitude of the electrical field. Compared to the so far reported alternatives with insulated electrodes, the here developed thermoplastic microfluidic bioreactors with uninsulated electrodes provide a new, versatile, and durable platform for in vitro cell studies toward the improvement of anti-cancer therapies including personalized treatment. [ABSTRACT FROM AUTHOR]

This paper presents the use of a commercial quartz crystal microbalance (QCM) to investigate live-cell activity in water-based toxic solutions. The QCM used in this research has a resonant frequency of 10 MHz and consists of an AT-cut quartz crystal with gold electrodes on both sides. This QCM was transformed into a functional biosensor by integrating with polydimethylsiloxane culturing chambers. Rainbow trout gill epithelial cells were cultured on the resonators as a sensorial layer. The fluctuation of the resonant frequency, due to the change of cell morphology and adhesion, is an indicator of water toxicity. The shift in the resonant frequency provides information about the viability of the cells after exposure to toxicants. The toxicity result shows distinct responses after exposing cells to 0.526 μM of pentachlorophenol (PCP) solution, which is the Military Exposure Guidelines concentration. This research demonstrated that the QCM is sensitive to a low concentration of PCP and no further modification of the QCM surface was required. [ABSTRACT FROM AUTHOR]

The electronic transport properties of single [2,2]paracyclophane molecules directly connected to gold and platinum electrodes have been investigated both theoretically and experimentally by using first-principles quantum transport simulations and break-junction experiments. For comparison, investigations on [3,3]- and [4,4]-paracyclophanes have also been performed. Our calculations show that the strength of the π-π interaction in paracyclophanes is critically dependent on the inter-ring distance. In contrast to [4,4]paracyclophane in which the π-π interaction is very weak due to the large inter-ring distance, the π-π interaction in [2,2]- and [3,3]-paracyclophanes is rather strong and dominates the electronic transport properties. In particular, for the asymmetric Au-[2,2]paracyclophane-Au junction in which the [2,2]paracyclophane molecule is connected to each gold electrode through a Au adatom and the two Au adatoms are attached in η1-fashion to two carbon atoms in the benzene backbones connecting with different ethylene groups, the transmission coefficient at the Fermi level is calculated to be 1.0 × 10-2, in excellent agreement with experiments. When the gold electrodes are replaced by platinum, the calculated transmission coefficient at the Fermi level of the symmetric Pt-[2,2]paracyclophane-Pt junction with one Pt adatom used as the linker group is increased to 0.83, demonstrating that the π-π stacking in [2,2]paracyclophane is efficient for electron transport when the molecule-electrode interfaces are electronically transparent. This is confirmed by our preliminary experimental studies on the Pt-[2,2]paracyclophane-Pt junctions, for which the low-bias junction conductance has reached 0.40 ± 0.02 G0 (G0 is the conductance quantum). These findings are helpful for the design of molecular electronic devices incorporating π-π stacking molecular systems. [ABSTRACT FROM AUTHOR]

We examine theoretically the effects of the bonding geometries at the gold-thiol interfaces on the inelastic tunneling spectra of propanedithiolate (PDT) molecules bridging gold electrodes and show that inelastic tunneling spectroscopy combined with theory can be used to determine these bonding geometries experimentally. With the help of density functional theory, we calculate the relaxed geometries and vibrational modes of extended molecules each consisting of one or two PDT molecules connecting two gold nanoclusters. We formulate a perturbative theory of inelastic tunneling through molecules bridging metal contacts in terms of elastic transmission amplitudes, and use this theory to calculate the inelastic tunneling spectra of the gold-PDT-gold extended molecules. We consider PDT molecules with both trans and gauche conformations bound to the gold clusters at top, bridge, and hollow bonding sites. Comparing our results with the experimental data of Hihath et al. [Nano Lett. 8, 1673 (2008)], we identify the most frequently realized conformation in the experiment as that of trans molecules top-site bonded to both electrodes. We find the switching from the 42 meV vibrational mode to the 46 meV mode observed in the experiment to be due to the transition of trans molecules from mixed top-bridge to pure top-site bonding geometries. Our results also indicate that gauche molecular conformations and hollow site bonding did not contribute significantly to the experimental inelastic tunneling spectra. For pairs of PDT molecules connecting the gold electrodes in parallel we find total elastic conductances close to twice those of single molecules bridging the contacts with similar bonding conformations and small splittings of the vibrational mode energies for the modes that are the most sensitive to the molecule-electrode bonding geometries. [ABSTRACT FROM AUTHOR]