Single- and multigrain nanojunctions with a self-assembled monolayer of conjugated molecules

Temperature-Dependent Charge Transport through Individually Contacted DNA Origami-Based Au Nanowires

DNA origami nanostructures have been used extensively as scaffolds for numerous applications such as for organizing both organic and inorganic nanomaterials, studying single molecule reactions, and fabricating photonic devices. Yet, little has been done toward the integration of DNA origami nanostructures into nanoelectronic devices. Among other challenges, the technical difficulties in producing well-defined electrical contacts between macroscopic electrodes and individual DNA origami-based nanodevices represent a serious bottleneck that hinders the thorough characterization of such devices. Therefore, in this work, we have developed a method to electrically contact individual DNA origami-based metallic nanowires using electron beam lithography. We then characterize the charge transport of such nanowires in the temperature range from room temperature down to 4.2 K. The room temperature charge transport measurements exhibit ohmic behavior, whereas at lower temperatures, multiple charge transport mechanisms such as tunneling and thermally assisted transport start to dominate. Our results confirm that charge transport along metallized DNA origami nanostructures may deviate from pure metallic behavior due to several factors including partial metallization, seed inhomogeneities, impurities, and weak electronic coupling among AuNPs. Besides, this study further elucidates the importance of variable temperature measurements for determining the dominant charge transport mechanisms for conductive nanostructures made by self-assembly approaches.

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.

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.

Tuning magnetic anisotropies of Fe films on Si(111) substrate via direction variation of heating current

We adopted a novel method to tune the terrace width of Si(111) substrate by varying the direction of heating current. It was observed that the uniaxial magnetic anisotropy (UMA) of Fe films grown on the Si(111) substrate enhanced with decreasing the terrace width and superimposed on the weak six-fold magnetocrystalline anisotropy. Furthermore, on the basis of the scanning tunneling microscopy (STM) images, self-correlation function calculations confirmed that the UMA was attributed mainly from the long-range dipolar interaction between the spins on the surface. Our work opens a new avenue to manipulate the magnetic anisotropy of magnetic structures on the stepped substrate by the decoration of its atomic steps.

Electrochemical fabrication of highly stable redox-active nanojunctions

Redox-gated molecular junctions were obtained starting with a relatively large gap between two electrodes, in the micrometer range, followed by electrochemical polymerization of aniline. Polyaniline (PANI) grows from the tip side until it bridges the two electrodes. The resulting junctions were characterized electrochemically by following the variation of the tip-substrate current as a function of the electrochemical gate potential for various bias voltages and by recording their I(V) characteristics. The two electrodes make contact through PANI wires, and microjunctions with conductances around 10(-3) S were obtained. On the basis of a similar setup, PANI nanojunctions with conductances between 10(-7) and 10(-8) S were made, where the current appears to be controlled by fewer than 10 oligoaniline strands. Despite the small number of strands connecting the two electrodes, the junctions are highly stable even when several successive potential sweeps are performed. Comparison of the conductance measured in the oxidized and reduced states leads to an on/off ratio of about 70-100, which is higher than that reported for a single aniline heptamer bridging two electrodes, highlighting the interest of connecting a few tens of molecules using the scanning electrochemical microscopy (SECM) configuration. In some cases, the switching of the PANI takes place in several individual conductance steps close to that obtained for a single oligoaniline. Finally, starting with a microjunction and mechanically withdrawing the tip shrinks it down to the nanometer scale and makes it possible to reach the regime where the conductance is controlled by a limited number of strands. This work presents an easy method for making redox-gated nanojunctions and for probing the conductance of a few oligoanilines despite an initially large tip-substrate gap.

Molecular electronic devices based on single-walled carbon nanotube electrodes

As the top-down fabrication techniques for silicon-based electronic materials have reached the scale of molecular lengths, researchers have been investigating nanostructured materials to build electronics from individual molecules. Researchers have directed extensive experimental and theoretical efforts toward building functional optoelectronic devices using individual organic molecules and fabricating metal-molecule junctions. Although this method has many advantages, its limitations lead to large disagreement between experimental and theoretical results. This Account describes a new method to create molecular electronic devices, covalently bridging a gap in a single-walled carbon nanotube (SWNT) with an electrically functional molecule. First, we introduce a molecular-scale gap into a nanotube by precise oxidative cutting through a lithographic mask. Now functionalized with carboxylic acids, the ends of the cleaved carbon nanotubes are reconnected with conjugated diamines to give robust diamides. The molecular electronic devices prepared in this fashion can withstand and respond to large environmental changes based on the functional groups in the molecules. For example, with oligoanilines as the molecular bridge, the conductance of the device is sensitive to pH. Similarly, using diarylethylenes as the bridge provides devices that can reversibly switch between conjugated and nonconjugated states. The molecular bridge can perform the dual task of carrying electrical current and sensing/recognition through biological events such as protein/substrate binding and DNA hybridization. The devices based on DNA can measure the difference in electrical properties of complementary and mismatched strands. A well-matched duplex DNA 15-mer in the gap exhibits a 300-fold lower resistance than a duplex with a GT or CA mismatch. This system provides an ultrasensitive way to detect single-nucleotide polymorphisms at the individual molecule level. Restriction enzymes can cleave certain cDNA strands assembled between the SWNT electrodes; therefore, these strands maintain their native conformation when bridging the ends of the SWNTs. This methodology for creating novel molecular circuits forges both literal and figurative connections between chemistry, physics, materials science, and biology and promises a new generation of integrated multifunctional sensors and devices.

Ballistic emission microscopy studies on metal-molecule interfaces

Ballistic electron emission microscopy (BEEM) experiments on metal-molecule interfaces are briefly reviewed. Results of BEEM experiments with two different orientations of molecules are presented and discussed. Significant differences in uniformity of transport through the molecular layer are found. Implications for device applications are briefly discussed.

Measurement of Single-Molecule Conductance

What is the conductance of a single molecule? This basic and seemingly simple question has been a difficult one to answer for both experimentalists and theorists. To determine the conductance of a molecule, one must wire the molecule reliably to at least two electrodes. The conductance of the molecule thus depends not only on the intrinsic properties of the molecule, but also on the electrode materials. Furthermore, the conductance is sensitive to the atomic-level details of the molecule-electrode contact and the local environment of the molecule. Creating identical contact geometries has been a challenging experimental problem, and the lack of atomic-level structural information of the contacts makes it hard to compare calculations with measurements. Despite the difficulties, researchers have made substantial advances in recent years. This review provides an overview of the experimental advances, discusses the advantages and drawbacks of different techniques, and explores remaining issues.

Fabrication of Steady Junctions Consisting of α,ω-Bis(thioacetate) Oligo(p-phenylene vinylene)s in Nanogap Electrodes

For obtaining molecular devices using metal-molecule-metal junctions, it is necessary to fabricate a steady conductive bridge-structure; that is stable chemical bonds need to be established from a single conductive molecule to two facing electrodes. In the present paper, we show that the steadiness of a conductive bridge-structure depends on the molecular structure of the bridge molecule for nanogap junctions using three types of modified oligo(phenylene vinylene)s (OPVs): alpha,omega-bis(thioacetate) oligo(phenylene vinylene) (OPV1), alpha,omega-bis(methylthioacetate) oligo(phenylene vinylene) (OPV2), and OPV2 consisting of ethoxy side chains (OPV3). We examined the change in resistance between the molecule-bridged junction and a bare junction in each of the experimental Au-OPV-Au junctions to confirm whether molecules formed steady bridges. Herein, the outcomes of whether molecules formed steady bridges were defined in terms of three types of result; successful, possible and failure. We define the ratio of the number of successful junctions to the total number of experimental junctions as successful rate. A 60% successful rate for OPV3 was higher than for the other two molecules whose successful rates were estimated to be approximately 10%. We propose that conjugated molecules consisting of methylthioacetate termini and short alkoxy side chains are well suited for fabricating a steady conductive bridge-structure between two facing electrodes.

Covalently Bridging Gaps in Single-Walled Carbon Nanotubes with Conducting Molecules

Molecular electronics is often limited by the poorly defined nature of the contact between the molecules and the metal surface. We describe a method to wire molecules into gaps in single-walled carbon nanotubes (SWNTs). Precise oxidative cutting of a SWNT produces carboxylic acid–terminated electrodes separated by gaps of ≤10 nanometers. These point contacts react with molecules derivatized with amines to form molecular bridges held in place by amide linkages. These chemical contacts are robust and allow a wide variety of molecules to be tested electrically. In addition to testing molecular wires, we show how to install functionality in the molecular backbone that allows the conductance of the single-molecule bridges to switch with pH.

An 11 nm Molecular Wire that Switches Electrochemically between an Insulating and a Fully Conjugated Conducting State

[structure: see text] Molecular wires consisting of oligothiophenes and oligothiophenylethynes, ranging in length from 2 to 11 nm, are synthesized and examined by spectroscopy and electrochemistry. UV spectroscopy shows that the longest wires are not fully conjugated, but twisted. However, cyclic voltammetry indicates that their monocations are now flattened and conjugated over the entire length range. In appropriate systems, the wires will link to two electrodes across a nanoscale gap and conduct current.

Monolayer Structures of Highly Photoluminescent Furan Oligoaryls: An Approach to Improve Packing Crystallinity of Dithiolated Aromatics

We demonstrated that mono- and dithiolated furan-containing oligoaryls (II-IV, see Chart 2) can be successfully synthesized via a one-pot strategy starting from propargylic dithioacetals. IRAS (infrared reflection-absorption spectroscopy) and STM (scanning tunneling microscopy) experiments revealed that single-component monolayers of II, III, and IV are essentially disordered, an important property that prevents excited photoluminescent molecules from self-quenching in the organic layers of an OLED device. Surprisingly, localized lattice packing of crystalline dithiolated furan oligoaryls on Au(111) can be assembled by immersing preadsorbed n-dodecanethiol SAMs in the corresponding deposition solutions. The discrepancy in the formation of disordered or localized crystalline structures is discussed. For single-component monolayers, the facile formation of S-Au bonds generates chaotically distributed monolayers in which the arched molecules hinge each other and block the desorptive pathways. The absence of crystalline packing is mainly attributed to the difficulty for the dithiols to simultaneously break two S-Au bonds, to desorb, and then to readsorb, the key step to improve the intermolecular attractions for crystalline SAMs. By preassembling n-dodecanethiol SAMs, the space for dithiolated compounds III and IV to adsorb is limited to domain boundaries or packing defects where crystalline packing of III and IV can grow.

Oligothiophene Isocyanides for Platinum-Based Molecular Electronic Applications

Understanding molecular orientation on a metal surface is key to designing molecular electronic device junctions. Though platinum device electrodes are of particular interest as a more stable alternative to the often used gold electrodes, the chemisorption of conducting molecules onto platinum surfaces has not been thoroughly studied. We present herein the first detailed study of the ability and manner in which soluble oligothiophene isocyanides, of lengths ranging from 2 to 7 nm, chemisorb onto platinum surfaces and nanoparticles. It was found that these oligothiophene isocyanides stand at a 41 degrees angle from the platinum surface normal, suggesting their applicability in molecule-bridged platinum electrode devices.