Resistance switch employing a simple metal nanogap junction

In recent years, several researchers have reported the occurrence of reversible resistance switching effects in simple metal nanogap junctions. A large negative resistance is observed in the I-V characteristics of such a junction when high-bias voltages are applied. This phenomenon is characteristic behaviour on the nanometre scale; it only occurs for gap widths slightly under 13?nm. Furthermore, such a junction exhibits a non-volatile resistance hysteresis when the bias voltage is reduced very rapidly from a high level to around 0?V, and when the bias voltage is reduced slowly. This non-volatile resistance change occurs as a result of changes in the gap width between the metal electrodes, brought about by the applied bias voltage.     

Molecular Signatures in the Transport Properties of Molecular Wire Junctions: What Makes a Junction “Molecular”?

The simplest component of molecular electronics consists of a single-molecule transport junction: a molecule sandwiched between source and drain electrodes, with or without a third gate electrode. In this Concept article, we focus on how molecules control transport in metal-electrode molecular junctions, and where the molecular signatures are to be found. In the situation where the molecule is relatively short and the gap between injection energy and molecular eigenstates is large, transport occurs largely by elastic tunneling, stochastic switching is common, and the vibronic signature can be found using inelastic electron tunneling spectroscopy (IETS). As the energy gaps for injection become smaller, one begins to see stronger molecular signatures - these include Franck-Condon-like structures in the current/voltage characteristic and strong vibronic interactions, which can lead to hopping behavior at the polaron limit. Conformational changes induced by the strong electric field lead to another strong manifestation of the molecular nature of the junction. We overview some of this mechanistic landscape, focusing on significant effects of switching (both stochastic and controlled by the electric field) and of molecular vibronic coupling.     

Optical switching of electric charge transfer pathways in porphyrin: a light-controlled nanoscale current router

We introduce a novel molecular junction based on a thiol-functionalized porphyrin derivative with two almost energetically degenerate equilibrium configurations. We show that each equilibrium structure defines a pathway of maximal electric charge transfer through the molecular junction and that these two conduction pathways are spatially orthogonal. We further demonstrate computationally how to switch between the two equilibrium structures of the compound by coherent light. The optical switching mechanism is presented in the relevant configuration subspace of the compound, and the corresponding potential and electric dipole surfaces are obtained by ab initio methods. The laser-induced isomerization takes place in two steps in tandem, while each step is induced by a two-photon process. The effect of metallic electrodes on the electromagnetic irradiation driving the optical switching is also investigated. Our study demonstrates the potential for using thiol-functionalized porphyrin derivatives for the development of a light-controlled nanoscale current router.     

Tuning the conductance of a molecular switch

The ability to control the conductance of single molecules will have a major impact in nanoscale electronics. Azobenzene, a molecule that changes conformation as a result of a trans/cis transition when exposed to radiation, could form the basis of a light-driven molecular switch. It is therefore crucial to clarify the electrical transport characteristics of this molecule. Here, we investigate, theoretically, charge transport in a system in which a single azobenzene molecule is attached to two carbon nanotubes. In clear contrast to gold electrodes, the nanotubes can act as true nanoscale electrodes and we show that the low-energy conduction properties of the junction may be dramatically modified by changing the topology of the contacts between the nanotubes and the molecules, and/or the chirality of the nanotubes (that is, zigzag or armchair). We propose experiments to demonstrate controlled electrical switching with nanotube electrodes.     

利用原子力显微镜改变P(VDF/TrFE)聚合物分子链偶极子极性实现数据存储的读/写/擦除/改写

包括读、写、擦除和改写等信息存储现象的产生是由于在外加电压的作用下，铁电体“聚偏二氟乙烯-三氟胸苷”异分子聚合物的分子链偶极子的极性发生了变化．为达此目的，使原子力显微镜工作在压电响应模式下，它比传统的数据存储方法要有优势．将20nm厚的“聚偏二氟乙烯-三氟胸苷”薄膜放置于原子力显微镜的导电悬臂梁触针和石墨基片之间，直流电通过AFM触针在几微秒的时间内极化铁电畴，同时用交流电来辨识这些纳米尺度(约60nm)的铁电畴，这些铁电畴的尺寸由外加电压的大小和持续时间等参数来决定，可以实现约19．2Gb／cm^2(120Gb／in^2)的数据存储．     

Coupled surface-enhanced Raman spectroscopy and electrical conductivity measurements of 1,4-phenylene diisocyanide in molecular electronic junctions

Probing the structure of molecules in a metal-molecule-metal junction under an applied voltage is critical for understanding molecular electron transport properties. We present an approach that allows recording surface-enhanced Raman spectra simultaneously with electrical measurements of a monolayer of molecules in molecular electronic junctions. 1,4-Phenylene diisocyanide in two different types of junctions was used to illustrate the approach. The results show that the molecular integrity was intact in the molecular junctions and under the applied bias. The monolayer sensitivity of the approach provides a new powerful tool for characterizing molecular structure in a molecular electronic junction.     

Environmental control of single-molecule junction transport

The conductance of individual 1,4-benzenediamine (BDA)-Au molecular junctions is measured in different solvent environments using a scanning tunneling microscope based point-contact technique. Solvents are found to increase the conductance of these molecular junctions by as much as 50%. Using first principles calculations, we explain this increase by showing that a shift in the Au contact work function is induced by solvents binding to undercoordinated Au sites around the junction. Increasing the Au contact work function reduces the separation between the Au Fermi energy and the highest occupied molecular orbital of BDA in the junction, increasing the measured conductance. We demonstrate that the solvent-induced shift in conductance depends on the affinity of the solvent to Au binding sites and also on the induced dipole (relative to BDA) upon adsorption. Via this mechanism, molecular junction level alignment and transport properties can be statistically altered by solvent molecule binding to the contact surface.     

Switching of current channels and new mechanism of magnetoresistance in a tunneling structure

We have experimentally studied the transport properties of a planar La_0.7Sr_0.3MnO₃ (LSMO)/Mn-depleted LSMO/MnSi tunneling structure, in which the Mn-depleted LSMO layer plays the role of a potential barrier between the conducting layers of LSMO and MnSi. The measurements were performed in geometry with the current direction parallel to the planes of interfaces in the tunneling structure. It is established that the structure exhibits a nonlinear current-voltage characteristic and possesses a positive magnetoresistance, the value of which depends on the tunneling current. It is suggested that specific features of the transport properties of this structure are related to the phenomenon of current channel switching between the conducting layers. The switching mechanism is based on the dependence of the resistance of the tunneling junction between the conducting layers on the bias voltage and the applied magnetic field.     

Study of electron-phonon interactions in a single molecule covalently connected to two electrodes

Presented here is a study of electron-phonon interactions in a single molecule junction where the molecule is covalently connected to two electrodes. In this system, vibration modes in a single molecule junction are measured by sweeping the bias voltage between the two electrodes and recording the differential conductance while the strain in the junction is changed by separating the two electrodes. This unique approach allows changes in conductance to be compared to changes in the configuration of a single molecule junction. This system opens a new door for characterizing single molecule junctions and a better understanding of the relationship between molecular conductance, electron-phonon interactions, and configuration.     

Mechanically controlled molecular orbital alignment in single molecule junctions

Research in molecular electronics often involves the demonstration of devices that are analogous to conventional semiconductor devices, such as transistors and diodes, but it is also possible to perform experiments that have no parallels in conventional electronics. For example, by applying a mechanical force to a molecule bridged between two electrodes, a device known as a molecular junction, it is possible to exploit the interplay between the electrical and mechanical properties of the molecule to control charge transport through the junction. 1,4'-Benzenedithiol is the most widely studied molecule in molecular electronics, and it was shown recently that the molecular orbitals can be gated by an applied electric field. Here, we report how the electromechanical properties of a 1,4'-benzenedithiol molecular junction change as the junction is stretched and compressed. Counterintuitively, the conductance increases by more than an order of magnitude during stretching, and then decreases again as the junction is compressed. Based on simultaneously recorded current-voltage and conductance-voltage characteristics, and inelastic electron tunnelling spectroscopy, we attribute this finding to a strain-induced shift of the highest occupied molecular orbital towards the Fermi level of the electrodes, leading to a resonant enhancement of the conductance. These results, which are in agreement with the predictions of theoretical models, also clarify the origins of the long-standing discrepancy between the calculated and measured conductance values of 1,4'-benzenedithiol, which often differ by orders of magnitude.     

Silicon-based molecular switch junctions

In contrast to the static operations of conventional semiconductor devices, the dynamic conformational freedom in molecular devices opens up the possibility of using individual molecules as new types of devices such as a molecular conformational switch or for molecular data storage. Bistable molecules—such as those having two stable cis and trans isomeric configurations—could provide, once clamped between two electrodes, a switching phenomenon in the non-equilibrium current response. Here, we model molecular switch junctions formed at silicon contacts and demonstrate the potential of such tunable molecular switches in electrode/molecule/electrode configurations. Using the non-equilibrium Green function (NEGF) approach implemented with the density-functional-based tight-binding (DFTB) theory, a series of properties such as electron transmissions, current-voltage characteristics in the different isomer conformations, and potential energy surfaces (PESs) as a function of the reaction coordinates along the trans to cis transition were calculated for two azobenzene-based model compounds. Furthermore, in order to investigate the stability of molecular switches under ambient conditions, molecular dynamics (MD) simulations at room temperature were performed and time-dependent fluctuations of the conductance along the MD pathways were calculated. Our numerical results show that the transmission spectra of the cis isomers are more conductive than trans counterparts inside the bias window for both model compounds. The current voltage characteristics consequently show the same trends. Additionally, calculations of the time-dependent transmission fluctuations along the MD pathways have shown that the transmission in the cis isomers is always significantly larger than that in their trans counterparts, showing that molecular switches can be expected to work as robust molecular switching components. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

Designing bistable [2]rotaxanes for molecular electronic devices

The development of molecular electronic components has been accelerated by the promise of increased circuit densities and reduced power consumption. Bistable rotaxanes have been assembled into nanowire crossbar devices, where they may be switched between low- and high-conductivity states, forming the basis for a molecular memory. These memory devices have been scaled to densities of 10(11) bits cm(-2), the 2020 node for memory of the International Technology Roadmap for Semiconductors. Investigations of the kinetics and thermodynamics associated with the electromechanical switching processes of several bistable [2]rotaxane derivatives in solution, self-assembled monolayers on gold, polymer electrolyte gels and in molecular switch tunnel junction devices are consistent with a single, universal switching mechanism whose speed is dependent largely on the environment, as well as on the structure of the switching molecule. X-ray reflectometry studies of the bistable rotaxanes assembled into Langmuir monolayers also lend support to an oxidatively driven mechanical switching process. Structural information obtained from Fourier transform reflection absorption infrared spectroscopy of rotaxane monolayers taken before and after evaporation of a Ti top electrode confirmed that the functionality responsible for switching is not affected by the metal deposition process. All the considerable experimental data, taken together with detailed computational work, support the hypothesis that the tunnelling current hysteresis, which forms the basis of memory operation, is a direct result of the electromechanical switching of the bistable rotaxanes.     

Reversible bistable switching in nanoscale thiol-substituted oligoaniline molecular junctions

Single molecular monolayers of oligoaniline dimers were integrated into sub-40-nm-diameter metal nanowires to form in-wire molecular junctions. These junctions exhibited reproducible room temperature bistable switching with zero-bias high- to low-current state conductance ratios of up to 50, switching threshold voltages of approximately +/-1.5 V, and no measurable decay in the high-state current over 22 h. Such switching was not observed in similarly fabricated saturated dodecane (C12) or conjugated oligo(phenylene ethynylene) (OPE) molecular junctions. The low- and high-state current versus voltage was independent of temperature (10-300 K), suggesting that the dominant transport mechanism in these junctions is coherent tunneling. Inelastic electron tunneling spectra collected at 10 K show a change in the vibrational modes of the oligoaniline dimers when the junctions are switched from the low- to the high-current state. The results of these measurements suggest that the switching behavior is an inherent molecular feature that can be attributed to the oligoaniline dimer molecules that form the junction.     

Computation of inception voltage and inception time of positive impulse corona in rod?plane gaps

A method is proposed for computing the inception voltage and time of the corona in air in rod-plane gaps under a positive switching impulse and ramp-shaped voltages. The method is based on the rate of natural production of free electrons in the atmosphere as a result of cosmic ray activity, local radioactivity or ultraviolet radiation from the sun. The computed inception voltages agree reasonably with those measured experimentally for different values of the steepness of the applied switching impulse and ramp-shaped voltages and also with those measured experimentally for a rod gap stressed by a switching impulse voltage. The method is applied to different rod-plane gaps with varying rod radius and gap spacing     

First principle study of the self‐switching characteristics of the guanine based single optical molecular switch using carbon nanotube electrodes

The switching property of an optical single molecular switch based on a single DNA molecule guanine with a single walled carbon nanotube electrode has been investigated using density functional theory along with non‐equilibrium Green''s function based first principle approach. The semi‐empirical model of this single bio‐molecular switch has been operated at an ultra‐high 25 THz frequency in mid‐UV range. This single bio‐molecule comprises switching activity upon UV photo‐excitation. The influence of the highest occupied molecular orbital and lowest unoccupied molecular orbital gap and the quantum ballistic transmission into the switching activity are discussed in detail in this study. It has been observed that the maximum ON–OFF ratio, i.e. 327 is obtained at +0.8 V bias voltage. Theoretical results show that current through the twisted form is sufficiently larger than the straightened form, which recommends that this structure has smart prospective application in the future generation switching nanotechnology.Inspec keywords: molecular electronic states, density functional theory, ab initio calculations, DNA, organic compounds, molecular electronics, Green''s function methods, molecular biophysics, single‐wall carbon nanotubes, optical switches, orbital calculationsOther keywords: nonequilibrium Green''s function, semiempirical model, single bio‐molecular switch, UV photo‐excitation, lowest unoccupied molecular orbital gap, first principle study, single optical molecular switch, switching property, optical single molecular switch, single DNA molecule guanine, single walled carbon nanotube electrode, density functional theory, highest occupied molecular orbital gap, switching nanotechnology     

Nanoelectromechanical switch operating by tunneling of an entire C60 molecule

We present a solid state single molecule electronic device where switching between two states with different conductance happens predominantly by tunneling of an entire C60 molecule. This conclusion is based on a novel statistical analysis of approximately 10(5) switching events. The analysis yields (i) the relative contribution of tunneling, current induced heating and thermal fluctuations to the switching mechanism, (ii) the voltage dependent energy barrier (approximately 100-200 meV) separating the two states of the switch and (iii) the switching attempt frequency, omega0, corresponding to a 2.8 meV mode, which is most likely rotational.     

Tunnel Magnetoresistance of the Heterocyclic Molecular Junctions: A Green’s Function Approach

We explore the spin-dependent transport properties in a molecular junction model made of heterocyclic molecules such as poythiophene, polyfuran, and polypyrrole sandwiched between ferromagnetic three-dimensional electrodes, hereafter named FM/h-molecule/FM junction, based on a tight-binding model and a generalized Green??s function method in the Landauer?CBüttiker formalism. The coherent spin-dependent transport through the energy levels of heterocyclic molecules, the transmittance, current?Cvoltage (I?CV) characteristics, and the tunnel magnetoresistance (TMR) of the FM/h-molecule/FM junction are numerically investigated. It is found that the spin-dependent transport properties are importantly influenced by the heteroatoms in the heterocyclic molecules. It is shown that the TMR of the molecular junction can be quite large (over 69?%) depending on the applied voltage and the molecular field of the FM electrodes.     

Subnanosecond avalanche switching in high-voltage silicon diodes with abrupt and graded p-n junctions

The phenomenon of delayed avalanche breakdown in high-voltage silicon diodes has been studied for the first time using an experimental setup with specially designed resistive coupler as a part of a high-quality matched measuring tract. Three types of diode structures with identical geometric parameters and close stationary breakdown voltages within 1.1–1.3 kV have been studied, including p ⁺-n-n ⁺ structures with abrupt p-n junctions and two different p ⁺-p-n-n ⁺ structures with graded p-n junctions. Upon switching of all structures, a voltage step with an amplitude above 1 kV and a rise time of ～100 ps at a breakdown voltage of about 2 kV is formed in the load. However, switching to a state with low (～150 V) residual voltage has been observed only in the structures with an abrupt p-n junction, while the voltage in structures with graded junctions only decreased to a level of ～1 kV, which is close to the stationary breakdown voltage.     

Controlling single-molecule conductance through lateral coupling of π orbitals

In recent years, various single-molecule electronic components have been demonstrated. However, it remains difficult to predict accurately the conductance of a single molecule and to control the lateral coupling between the π orbitals of the molecule and the orbitals of the electrodes attached to it. This lateral coupling is well known to cause broadening and shifting of the energy levels of the molecule; this, in turn, is expected to greatly modify the conductance of an electrode-molecule-electrode junction. Here, we demonstrate a new method, based on lateral coupling, to mechanically and reversibly control the conductance of a single-molecule junction by mechanically modulating the angle between a single pentaphenylene molecule bridged between two metal electrodes. Changing the angle of the molecule from a highly tilted state to an orientation nearly perpendicular to the electrodes changes the conductance by an order of magnitude, which is in qualitative agreement with theoretical models of molecular π-orbital coupling to a metal electrode. The lateral coupling is also directly measured by applying a fast mechanical perturbation in the horizontal plane, thus ruling out changes in the contact geometry or molecular conformation as the source for the conductance change.     

Ultrafast current switching using the tunneling-assisted impact ionization front in a silicon semiconductor closing switch

Ultrafast current switching in semiconductors, based on the mechanism of tunneling-assisted impact ionization front, has been experimentally implemented and theoretically studied. A voltage pulse with an amplitude of 220 kV and a front duration of 1 ns was applied to a semiconductor device containing 20 serially connected silicon diode structures. After switching, 150-to 160-kV pulses with a power of 500 MW, a pulse duration of 1.4 ns, and a front duration of 200–250 ps were obtained in a 50-Ω transmission line. The maximum current and voltage buildup rates amounted to 10 kA/ns and 500 kV/ns, respectively, at a switched current density of 13 kA/cm². The results of numerical simulation are presented, which show that the current switching is initiated at a threshold field strength of about 1 MV/cm in the vicinity of the p-n junction, where the tunneling-assisted impact ionization begins.