Charge Transport Characteristics of Molecular Electronic Junctions Studied by Transition Voltage Spectroscopy.

The field of molecular electronics is prompted by tremendous opportunities for using a single-molecule and molecular monolayers as active components in integrated circuits. Until now, a wide range of molecular devices exhibiting characteristic functions, such as diodes, transistors, switches, and memory, have been demonstrated. However, a full understanding of the crucial factors that affect charge transport through molecular electronic junctions should yet be accomplished. Remarkably, recent advances in transition voltage spectroscopy (TVS) elucidate that it can provide key quantities for probing the transport characteristics of the junctions, including, for example, the position of the frontier molecular orbital energy relative to the electrode Fermi level and the strength of the molecule-electrode interactions. These parameters are known to be highly associated with charge transport behaviors in molecular systems and can then be used in the design of molecule-based devices with rationally tuned electronic properties. This article highlights the fundamental principle of TVS and then demonstrates its major applications to study the charge transport properties of molecular electronic junctions.

Effects of Natural Versus Synthetic Consonant and Vowel Stimuli on Cortical Auditory-Evoked Potential.

BACKGROUND AND OBJECTIVES: Natural and synthetic speech signals effectively stimulate cortical auditory evoked potential (CAEP). This study aimed to select the speech materials for CAEP and identify CAEP waveforms according to gender of speaker (GS) and gender of listener (GL). SUBJECTS AND METHODS: Two experiments including a comparison of natural and synthetic stimuli and CAEP measurement were performed of 21 young announcers and 40 young adults. Plosive /g/ and /b/ and aspirated plosive /k/ and /p/ were combined to /a/. Six bisyllables-/ga/-/ka/, /ga/-/ba/, /ga/-/pa/, /ka/-/ba/, /ka/-/pa/, and /ba/-/pa/-were formulated as tentative forwarding and backwarding orders. In the natural and synthetic stimulation mode (SM) according to GS, /ka/ and /pa/ were selected through the first experiment used for CAEP measurement. RESULTS: The correction rate differences were largest (74%) at /ka/-/ pa/ and /pa/-/ka/; thus, they were selected as stimulation materals for CAEP measurement. The SM showed shorter latency with P2 and N1-P2 with natural stimulation and N2 with synthetic stimulation. The P2 amplitude was larger with natural stimulation. The SD showed significantly larger amplitude for P2 and N1-P2 with /pa/. The GS showed shorter latency for P2, N2, and N1-P2 and larger amplitude for N2 with female speakers. The GL showed shorter latency for N2 and N1-P2 and larger amplitude for N2 with female listeners. CONCLUSIONS: Although several variables showed significance for N2, P2, and N1-P2, P1 and N1 did not show any significance for any variables. N2 and P2 of CAEP seemed affected by endogenous factors.

Electrostatic Gate Control in Molecular Transistors.

Molecular transistors, in which single molecules serve as active channel components in a three-terminal device geometry, constitute the building blocks of molecular scale electronic circuits. To demonstrate such devices, a gate electrode has been incorporated in several test beds of molecular electronics. The frontier orbitals' alignments of a molecular transistor can be delicately tuned by modifying the molecular orbital energy with the gate electrode. In this review, we described electrostatic gate control of solid-state molecular transistors. In particular, we focus on recent experimental accomplishments in fabrication and characterization of molecular transistors.

Charge Transport Characteristics of the PEDOT:PSS Interlayer Electrode/Molecule Physisorbed Contact in Large-Area Molecular Junctions.

Here we studied the effect of electrical contact between the PEDOT:PSS conducting polymer and the prototype benzenethiol molecules on the charge transport through the PEDOT:PSS-electrode molecular junctions. For this study, two different benzenethiol molecules were incorporated into the molecular junctions: 4-methylbenzenethiol (MBT) and 1,4-benzenedithiol (BDT). They have the same backbone structure but different top end-group. By investigating the MBT and BDT junctions with the PEDOT:PSS electrode, we demonstrated that the charge transport characteristics were dominated by distinct electrical contact properties at the PEDOT:PSS/molecule interface.

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.

Fabrication and Characterization of Molecular Electronic Devices.

The concept of molecular electronic devices is utilizing single molecules or molecular monolayers as active electronic components. Rapid advances in technology have enabled us to engineer molecular electronic devices with diverse functionalities. This review article emphasizes on experimental aspects of electronic devices made with single molecules or molecular monolayers, with a primary focus on the characterization and manipulation of charge transport.

Combing non-epitaxially grown nanowires for large-area electronic devices.

A facile route for aligning randomly oriented nanowires synthesized by a vapor-liquid-solid method for the fabrication of nanoelectronic devices was achieved using a polymer combing technique. By controlling the Young's modulus of the polymer combs, van der Waals interactions and shearing forces between the combs and nanowires can be manipulated and thus the nanowire density and alignment can be controlled. Using the proposed method, field-effect transistors were directly fabricated on as-grown substrates after aligning the nanowires, thereby demonstrating the feasibility of the scheme for the production of nanoelectronic devices.

Conductance and vibrational states of single-molecule junctions controlled by mechanical stretching and material variation.

The changes of molecular conformation, contact geometry, and metal-molecule bonding are revealed by inelastic-electron-tunneling spectroscopy measurements characterizing the molecular vibrational modes and the metal-phonon modes in alkanedithiol molecular junctions at low temperature. Combining inelastic-electron-tunneling spectroscopy with mechanical control and electrode material variation (Au or Pt) enables separating the influence of contact geometry and of molecular conformation. The mechanical strain of different electrode materials can be imposed onto the molecule, opening a new route for controlling the charge transport through individual molecules.

Single molecule electronic devices.

Single molecule electronic devices in which individual molecules are utilized as active electronic components constitute a promising approach for the ultimate miniaturization and integration of electronic devices in nanotechnology through the bottom-up strategy. Thus, the ability to understand, control, and exploit charge transport at the level of single molecules has become a long-standing desire of scientists and engineers from different disciplines for various potential device applications. Indeed, a study on charge transport through single molecules attached to metallic electrodes is a very challenging task, but rapid advances have been made in recent years. This review article focuses on experimental aspects of electronic devices made with single molecules, with a primary focus on the characterization and manipulation of charge transport in this regime.

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.

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.

Characterization of the tip-loading force-dependent tunneling behavior in alkanethiol metal-molecule-metal junctions by conducting atomic force microscopy.

We report on a tip-loading force-dependent tunneling behavior through alkanethiol self-assembled monolayers formed in metal-molecule-metal junctions, using conducting atomic force microscopy. The metal-molecule contacts were formed by placing a conductive tip in a stationary point contact on alkanethiol self-assembled monolayers under a controlled tip-loading force. Current-voltage characteristics in the alkanethiol junctions are simultaneously measured, while varying the loading forces. Tunneling current through the alkanethiol junctions increases and decay coefficient beta(N) decreases, respectively, with increasing tip-loading force, which results from enhanced intermolecular charge transfer in a tilted molecular configuration under the tip-loading effect.

Charge transport of alkanethiol self-assembled monolayers in micro-via hole devices.

In this paper we fabricated 13440 microscale via hole structure devices using different length of alkanethiol self-assembled monolayers and characterized their electronic transport properties. Statistically averaged transport parameters such as current density, transport barrier height, effective electron mass, and transport decay coefficient were obtained from the great number of these devices. The yield of working microdevices was found as 1.5%. Temperature variable current-voltage characteristics for alkanethiol micro-via hole devices showed typical tunneling behavior when properly fabricated.