Electrical characterization of benzenedithiolate molecular electronic devices with graphene electrodes on rigid and flexible substrates.

We investigated the electrical characteristics of molecular electronic devices consisting of benzenedithiolate self-assembled monolayers and a graphene electrode. We used the multilayer graphene electrode as a protective interlayer to prevent filamentary path formation during the evaporation of the top electrode in the vertical metal-molecule-metal junction structure. The devices were fabricated both on a rigid SiO2/Si substrate and on a flexible poly(ethylene terephthalate) substrate. Using these devices, we investigated the basic charge transport characteristics of benzenedithiolate molecular junctions in length- and temperature-dependent analyses. Additionally, the reliability of the electrical characteristics of the flexible benzenedithiolate molecular devices was investigated under various mechanical bending conditions, such as different bending radii, repeated bending cycles, and a retention test under bending. We also observed the inelastic electron tunneling spectra of our fabricated graphene-electrode molecular devices. Based on the results, we verified that benzenedithiolate molecules participate in charge transport, serving as an active tunneling barrier in solid-state graphene-electrode molecular junctions.

A new approach for high-yield metal-molecule-metal junctions by direct metal transfer method.

The realization of high-yield, stable molecular junctions has been a long-standing challenge in the field of molecular electronics research, and it is an essential prerequisite for characterizing and understanding the charge transport properties of molecular junctions prior to their device applications. Here, we introduce a new approach for obtaining high-yield, vertically structured metal-molecule-metal junctions in which the top metal electrodes are formed on alkanethiolate self-assembled monolayers by a direct metal transfer method without the use of any additional protecting interlayers in the junctions. The fabricated alkanethiolate molecular devices exhibited considerably improved device yields (∼70%) in comparison to the typical low device yields (less than a few %) of molecular junctions in which the top metal electrodes are fabricated using the conventional evaporation method. We compared our method with other molecular device fabrication methods in terms of charge transport parameters. This study suggests a potential new device platform for realizing robust, high-yield molecular junctions and investigating the electronic properties of devices.

Observation of molecular orbital gating.

The control of charge transport in an active electronic device depends intimately on the modulation of the internal charge density by an external node. For example, a field-effect transistor relies on the gated electrostatic modulation of the channel charge produced by changing the relative position of the conduction and valence bands with respect to the electrodes. In molecular-scale devices, a longstanding challenge has been to create a true three-terminal device that operates in this manner (that is, by modifying orbital energy). Here we report the observation of such a solid-state molecular device, in which transport current is directly modulated by an external gate voltage. Resonance-enhanced coupling to the nearest molecular orbital is revealed by electron tunnelling spectroscopy, demonstrating direct molecular orbital gating in an electronic device. Our findings demonstrate that true molecular transistors can be created, and so enhance the prospects for molecularly engineered electronic devices.

Successful Management of and Recovery from Multiple Cranial Nerve Palsies following Surgical Ventral Stabilization in a Dog with Atlantoaxial Subluxation.

A 4-year-old spayed female miniature poodle dog presented with a 1-week history of acute tetraparesis. A neurological examination revealed severe neck pain and non-ambulatory tetraparesis. Computed tomography and magnetic resonance imaging showed hypoplastic dens with moderate compression of the spinal cord at C1-C2. The atlantoaxial subluxation (AAS) was surgically stabilized using ventral pins and polymethylmethacrylate (PMMA) cement. On the second postoperative day, the patient showed significant dyspnea, and aspiration pneumonia was identified on radiography. The patient exhibited dysphagia with abnormal food prehension and an inability to protrude the tongue, with no gag reflex. We tentatively diagnosed the patient with multiple cranial nerve (CN) palsies involving the 9th, 10th, and 12th CNs following surgical ventral stabilization. The protruding cranial part of the implanted PMMA cement, which could mechanically contribute to the corresponding CNs dysfunction, was surgically removed. The symptoms gradually improved, and the patient showed normal tongue movement 1 month after revision surgery. In conclusion, we report herein a canine case of multiple CN palsies following ventral stabilization surgery for AAS. The experience gained from this case suggests an optimized management plan for postoperative neurological complications associated with ventral stabilization.

Positive-/negative-, erasable-/immobilized-, mono-/multi-color fluorescence image patterning of molecular-scale porous polymer film via a microcontact printing method using various chemical inks.

Fluorescent image patterns of a substituted acetylene polymer film with a large FFV were successfully obtained by a µCP method using several kinds of chemical ink compounds. PO and SCA generated positive-type fluorescent image patterns. On the other hand, an ethanolic solution of DNT generated a negative-type fluorescent image pattern due to a significant quenching effect. An NMP solution of NR gave a two-color image pattern due to an intermolecular energy transfer from PTMSDPA to NR.

Transport properties of coupled semiconductor quantum dots.

We have measured the electronic transport properties of the coupled quantum dot devices at low temperatures. The interplay between the strong many body spin interaction and the molecular states are probed in linear and non-linear transport regime. We observe the formation of strong coherent molecular states clearly visible in the double dot conductance phase diagram. In our study, the spin configuration in multiply coupled quantum dots could be identified using Kondo phenomenon. In addition, the characteristics of the spin dependent molecular states and phase dependant tunneling have been also observed using non-linear conductance measurement of the double dots. The results suggest the importance of the diverse spin related physical issues in artificial quantum dot devices.

Origin of multi-level switching and telegraphic noise in organic nanocomposite memory devices.

The origin of negative differential resistance (NDR) and its derivative intermediate resistive states (IRSs) of nanocomposite memory systems have not been clearly analyzed for the past decade. To address this issue, we investigate the current fluctuations of organic nanocomposite memory devices with NDR and the IRSs under various temperature conditions. The 1/f noise scaling behaviors at various temperature conditions in the IRSs and telegraphic noise in NDR indicate the localized current pathways in the organic nanocomposite layers for each IRS. The clearly observed telegraphic noise with a long characteristic time in NDR at low temperature indicates that the localized current pathways for the IRSs are attributed to trapping/de-trapping at the deep trap levels in NDR. This study will be useful for the development and tuning of multi-bit storable organic nanocomposite memory device systems.

Statistical investigation of the length-dependent deviations in the electrical characteristics of molecular electronic junctions fabricated using the direct metal transfer method.

We fabricated and analyzed the electrical transport characteristics of vertical type alkanethiolate molecular junctions using the high-yield fabrication method that we previously reported. The electrical characteristics of the molecular electronic junctions were statistically collected and investigated in terms of current density and transport parameters based on the Simmons tunneling model, and we determined representative current-voltage characteristics of the molecular junctions. In particular, we examined the statistical variations in the length-dependent electrical characteristics, especially the Gaussian standard deviation σ of the current density histogram. From the results, we found that the magnitude of the σ value can be dependent on the individual molecular length due to specific microscopic structures in the molecular junctions. The probable origin of the molecular length-dependent deviation of the electrical characteristics is discussed.

1/f Noise Scaling Analysis in Unipolar-Type Organic Nanocomposite Resistive Memory.

We studied noise characteristics of a nanocomposite of polyimide (PI) and phenyl-C61-butyric acid methyl ester (PCBM) (denoted as PI:PCBM), a composite for the organic nonvolatile resistive memory material. The current fluctuations were investigated over a bias range that covers various intermediate resistive states and negative differential resistance (NDR) in organic nanocomposite unipolar resistive memory devices. From the analysis of the 1/f(γ) type noises, scaling behavior between the relative noise power spectral density S̃ and resistance R was observed, indicating a percolating behavior. Considering a linear rate equation of the charge trapping-detrapping at traps, the percolation behavior and NDR could be understood by the modulation of the conductive phase fraction φ with an external bias. This study can enhance the understanding of the NDR phenomena in organic nanocomposite unipolar resistive memory devices in terms of the current path formation and the memory switching.

Noise characteristics of charge tunneling via localized states in metal--molecule--metal junctions.

We report the noise characteristics of charge transport through an alkyl-based metal--molecule--metal junction. Measurements of the 1/f noise, random telegraph noise, and shot noise demonstrated the existence of localized traps in the molecular junctions. These three noise measurements exhibited results consistent with trap-mediated tunneling activated over approximately 0.4 V by trapping and detrapping processes via localized states (or defects). The noise characterizations will be useful in evaluating the influences of localized states on charge transport in molecular or other electronic junctions.