Molecular Electronic Circuits Research Articles

Solution‐Processed Reduced Graphene Oxide Films as Electronic Contacts for Molecular Monolayer Junctions

In monolayer-based molecular electronics, functionalized molecules are utilized as active components; in electronic devices, these functionalities are accessed through stable electrical contacts. In such molecular junctions, a metallic contact can be used to complete a molecular electronic circuit. For practical applications, however, the formation of a soft contact is highly desired to avoid the possibility of a metal filamentary short circuit or molecular damage caused by the penetration of metal atoms into molecular monolayers during direct metal deposition. Therefore, contact fabrication techniques such as indirect metal evaporation, surfacediffusion-mediated deposition, and soft organic interlayer coating have been developed for molecular electronic devices. One successful technique used for the formation of stable molecular junctions involves the use of a conducting layer of an organic material such as single-walled nanotubes (SWNTs), a conducting polymer (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonic acid), PEDOT:PSS), or multilayered graphene as a top electrode or a bridging interfacial layer between the active molecules and a top electrode. These layers can provide proper contact resistance to facilitate true molecular effects in monolayer-based devices composed of alkanethiols/alkanedithiols, photoisomers, p-conjugated organic molecules, or metal complexes. Nonetheless, although molecule-dependent electronic transport has been thoroughly investigated, previous interlayer junctions have not allowed for a clear elucidation of the intrinsic properties of functionalized molecules such as memory components in molecular monolayer devices. A reliable device system with a highly sensitive interlayer with molecular functionality is required for the further development of these molecular devices. To develop a highly sensitive interlayer with a high device yield, it is crucial to create an interlayer that is stable, both chemically and electrically, with a soft contact that is able to communicate between the functional molecular monolayer and a metal electrode. To optimize the sensitivity of the interlayer, its thickness should be easily controllable by varying the number of layers. For a stable junction, the interlayer should protect the molecular monolayers from the penetration of hot metal nanoparticles during the vapor deposition of a metallic top electrode because most functional molecular monolayers, which are only a few nanometres thick (< 2–3 nm), are vulnerable to this contamination. Chemically exfoliated graphene oxide (GO) consists of atomically thin sheets of oxidized graphite that are dispersible in various solvents and can be used to produce dispersible reduced graphene oxide (rGO) by chemical reduction. The conductivity of rGO is comparable to that of SWNT and is increased by thermal treatment, similar to graphite. Graphene and rGO are composed of sp carbon networks and can act as electrodes for electronic devices. One notable advantage of the use of rGO for organic electronics is its solution processability compared with that of graphene. For example, an rGO film electrode can be easily prepared by spin-coating an rGO solution onto a substrate or by spincoating with GO followed by vapor reduction. Additionally, the strong p–p interactions between rGO nanosheets result in a graphite interlayer distance of approximately 0.34 nm, leading to a high conductivity that is comparable to that of highly oriented pyrolytic graphite (HOPG). Electrical conduction in rGO thin films is treated as that through a semimetal such as graphite, and the contact resistance in rGO devices is negligible. Because rGO has high chemical stability, mechanical strength, and a work function comparable to that of gold, rGO has been considered to be a promising candidate for interfacial electronic contacts in monolayer molecular electronics. Herein, we report the development of a solution-processed electronic contact between rGO thin films and molecular components in monolayer-based devices. The rGO contact allows for stable monolayer junctions of alkanethiol monolayers (e.g., molecular resistors) and redox-active metal complex monolayers (e.g., molecular memories) that prevent the formation of metallic short circuits and exhibit excellent junction preservation. The effects of monolayer thickness and molecular functionality were examined in novel molecular junctions using rGO interlayers. Through the semimetallic rGO interlayer contact, the current hysteresis loops and threshold conductance [*] Dr. S. Seo, M. Min, Dr. J. Lee, Dr. H. Lee National Creative Research Initiative, Center for Smart Molecular Memory, Department of Chemistry, Samsung-SKKU Graphene Center, Sungkyunkwan University 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do 440-746 (Korea) E-mail: hyoyoung@skku.edu

First-principles calculation on the conductance of ruthenium-quasi cumulene-ruthenium molecular junctions

The conductance of a family of ruthenium-quasi cumulene-ruthenium molecular junctions including different numbers of carbon atoms, both in even numbers and odd numbers, are investigated using a fully self-consistent ab initio approach which combines the non-equilibrium Green’s function formalism with density functional theory. Our calculations demonstrate that although the overall transport properties of the Ru-quasi cumulene-Ru junctions with an even number of carbon atoms are different from those of the junctions with an odd number of carbon atoms, the difference between the corresponding current-voltage (I–V) characteristics of these molecular junctions declines to lesser than 16% when the voltage goes up. In each group, the molecular junctions give a large transmission around the Fermi level since the Ru-C π bonds can extend the π conjugation of the carbon chains into the Ru electrodes, and their I–V characteristics are almost linear and independent of the chain length, illustrating potential applications as conducting molecular wires in future molecular electronic devices and circuits.

Bottom-up assembly of high density molecular nanowire cross junctions at a solid/liquid interface

A monoalkylated tetrathiafulvalene derivative forms multilayer structures at the solid-liquid interface, with high density of cross junctions, which are interesting for molecular electronic circuit self-assembly.

Controlled Deposition of Gold Nanowires on Semiconducting and Nonconducting Surfaces

One of the pressing problems in advancing nanoelectronic applications and systems is to develop a simple means of freely connecting at a nanometric level electronic components under ambient conditions without the need for vacuum or electron or ion beam operational steps. Such environments may have detrimental effects on the molecular or biomolecular constituents of molecular electronic circuits. Although there has been defined progress in connecting structures that are of nanometric dimension, new methods in this area of nanotechnology with general applicability add to the arsenal of tools for addressing this standing problem. This paper addresses freely placing under ambient conditions, with fountain pen nanolithography, a 120 nm dimension line of gold nanocolloids deposited with precise registration in a 100 nm gap between two 250 nm wide conducting electrodes patterned by electron beam lithography.

Defect-tolerant Logic with Nanoscale Crossbar Circuits

Crossbar architectures are one approach to molecular electronic circuits for memory and logic applications. However, currently feasible manufacturing technologies for molecular electronics introduc...

A 160-kilobit molecular electronic memory patterned at 1011 bits per square centimetre

The primary metric for gauging progress in the various semiconductor integrated circuit technologies is the spacing, or pitch, between the most closely spaced wires within a dynamic random access memory (DRAM) circuit. Modern DRAM circuits have 140 nm pitch wires and a memory cell size of 0.0408 mum(2). Improving integrated circuit technology will require that these dimensions decrease over time. However, at present a large fraction of the patterning and materials requirements that we expect to need for the construction of new integrated circuit technologies in 2013 have 'no known solution'. Promising ingredients for advances in integrated circuit technology are nanowires, molecular electronics and defect-tolerant architectures, as demonstrated by reports of single devices and small circuits. Methods of extending these approaches to large-scale, high-density circuitry are largely undeveloped. Here we describe a 160,000-bit molecular electronic memory circuit, fabricated at a density of 10(11) bits cm(-2) (pitch 33 nm; memory cell size 0.0011 microm2), that is, roughly analogous to the dimensions of a DRAM circuit projected to be available by 2020. A monolayer of bistable, [2]rotaxane molecules served as the data storage elements. Although the circuit has large numbers of defects, those defects could be readily identified through electronic testing and isolated using software coding. The working bits were then configured to form a fully functional random access memory circuit for storing and retrieving information.

Reversible Formation of Molecular Junctions in 2D Nanoparticle Arrays

ual junctions into functional electronic circuits remains a demanding task, requiring innovative approaches in fabrication philosophy and circuit structure. [12,13] We show here that an approach combining the self-assembly [14] and microcontact printing [15] of ligand-protected metallic nanoparticles, followed by an in situ ligand-exchange reaction, [16] allows the preparation of stable 2D networks of molecular junctions. [17] A significant decrease in resistance (up to three orders of magnitude) after the exchange of alkanethiol ligands with conjugated, double-ended organic wires (thiolated oligo(phenylene ethynylene), OPE) confirms a proper interlinking of neighboring nanoparticles. We also demonstrate that the formation of the molecular junctions is reversible, making nanoparticle networks a promising platform for the development of molecular electronic circuits. The flexibility of this approach lets us envisage the realization of more complex networks, for instance, by intermixing ensembles of bimodal nanoparticles [18] of different materials. Both experimental and theoretical evidence shows that the electronic properties of molecular junctions are not only dictated by the molecules contacted but also depend on the anchoring groups and the electrodes forming the junction. [3,11] The use of nanometer-sized metallic colloids as electrodes appears, therefore, as an elegant solution to build molecular junctions with well-defined geometry and electronic properties. Recent results on colloid dimers interlinked by a conjugated molecule demonstrate the validity of this approach. [19] The demonstration here of the fabrication of reversible 2D networks of hybrid metallic colloid organic-molecule junctions opens new possibilities for the realization of molecular circuits.

A molecular switch and electronic circuit modulate catalase activity in catalase‐peroxidases

The catalase reaction of catalase-peroxidases involves catalase-specific features built into a peroxidase core. An arginine, 20 A from the active-site heme, acts as a molecular switch moving between two conformations, one that activates heme oxidation and one that activates oxoferryl heme reduction by H(2)O(2), facilitating the catalatic pathway in a peroxidase. The influence of the arginine is imparted to the heme through its association with or dissociation from a tyrosinate that modulates reactivity through a Met-Tyr-Trp crosslinked adduct and a pi electron interaction of the heme with the adduct Trp.

Two Computational Primitives for Algorithmic Self-Assembly: Copying and Counting

Copying and counting are useful primitive operations for computation and construction. We have made DNA crystals that copy and crystals that count as they grow. For counting, 16 oligonucleotides assemble into four DNA Wang tiles that subsequently crystallize on a polymeric nucleating scaffold strand, arranging themselves in a binary counting pattern that could serve as a template for a molecular electronic demultiplexing circuit. Although the yield of counting crystals is low, and per-tile error rates in such crystals is roughly 10%, this work demonstrates the potential of algorithmic self-assembly to create complex nanoscale patterns of technological interest. A subset of the tiles for counting form information-bearing DNA tubes that copy bit strings from layer to layer along their length.

Integrated multiscale modeling of molecular computing devices

Molecular electronics, in which single organic molecules are designed to perform the functions of transistors, diodes, switches and other circuit elements used in current siliconbased microelecronics, is drawing wide interest as a potential replacement technology for conventional silicon-based lithographically etched microelectronic devices. In addition to their nanoscopic scale, the additional advantage of molecular electronics devices compared to silicon-based lithographically etched devices is the promise of being able to produce them cheaply on an industrial scale using wet chemistry methods (i.e., self-assembly from solution). The design of molecular electronics devices, and the processes to make them on an industrial scale, will require a thorough theoretical understanding of the molecular and higher level processes involved. Hence, the development of modeling techniques for molecular electronics devices is a high priority from both a basic science point of view (to understand the experimental studies in this field) and from an applied nanotechnology (manufacturing) point of view. Modeling molecular electronics devices requires computational methods at all length scales – electronic structure methods for calculating electron transport through organic molecules bonded to inorganic surfaces, molecular simulation methods for determining the structure of self-assembled films of organic molecules on inorganic surfaces, mesoscale methods to understand and predict the formation of mesoscale patterns on surfaces (including interconnect architecture), and macroscopic scale methods (including finite element methods) for simulating the behavior of molecular electronic circuit elements in a larger integrated device. Here we describe a large Department of Energy project involving six universities and one national laboratory aimed at developing integrated multiscale methods for modeling molecular electronics devices. The project is funded equally by the Office of Basic Energy Sciences and the Office of Advanced Scientific Computing Research, both within the Office of Science of the Department of Energy.

From DNA to transistors

The rapid advance in molecular biology and nanotechnology opens up the possibility to explore the interface between biology and electronics at the single-molecule level. We focus on the organization of molecular electronic circuits. Interconnecting an immense number of molecular devices into a functional circuit and constructing a framework for integrated molecular electronics requires new concepts. A promising avenue relies on bottom-up assembly where the information for the circuit connectivity and functionality is embedded in the molecular building blocks. Biology can provide concepts and mechanisms for advancing this approach, but there is no straightforward way to apply them to electronics since biological molecules are essentially electrically insulating. Bridging the chasm between biology and electronics therefore presents great challenges. Circuit organization on the molecular scale is considered and contrasted with the levels of organization presented by the living world. The discussion then focuses on our proposal to harness DNA and molecular biology to construct the scaffold for integrated molecular electronics. DNA metallization is used to convert the DNA scaffold into a conductive one. We present the framework of sequence-specific molecular lithography based on the biological mechanism of homologous genetic recombination and carried out by the bacterial protein RecA. Molecular lithography enables us to use the information encoded in the scaffold DNA molecules for directing the construction of an electronic circuit. We show that it can lead all the way from DNA molecules to working transistors in a test-tube. Carbon nanotubes are incorporated as the active electronic components in the DNA-templated transistors. Our approach can, in principle, be applied to the fabrication of larger-scale electronic circuits. The realization of complex DNA-based circuits will, however, require new concepts and additional biological machinery allowing, for example, feedback from the electronic functionality to direct the assembly process and adaptation mechanisms.

Fabrication of molecular-electronic circuits by nanoimprint lithography at low temperatures and pressures

We have utilized a modified version of thermal nanoimprint lithography to fabricate a rewritable, nonvolatile, molecular memory device with a density of 6.4 Gbit/cm2. It has the advantages of a relatively low operating temperature of (∼70 °C) and pressure of (<500 psi or 4.5 MPa), both of which are critical to preserving the integrity of the molecular layer. The architecture of the circuit was based on an 8×8 crossbar structure, with an active molecular layer sandwiched between the top and bottom electrodes. A liftoff process was utilized to produce the top and bottom electrodes made of Pt/Ti bilayers. The active molecular layer was deposited by the Languir–Blodgett technique. We utilized a new class of nanoimprint resist formulated by dissolving a polymer in its monomer. The formulation we used, was poly(benzyl methacrylate), dissolved in benzyl methacrylate with t-butyl peroxy 2-ethylhexanoate added as a self-initiator (8:90:2 by weight). The new resist allowed us to achieve Pt/Ti lines of 40 nm in width and 130 nm in pitch.

Molecular binding at gold transport interfaces. III. Field dependence of electronic properties.

The behavior of the electronic structure in a metal/molecular/metal junction as a function of the applied electric field is studied using density functional methods. Although the calculations reported here do not include the electrode bulk, or intermolecular interactions, and do not permit actual transport to occur, nevertheless they illuminate the charging, energy shift, polarization and orbital occupation changes in the molecular junction upon the application of a static electric field. Specifically, external electric fields generally induce polarization localization on the two cluster ends. The HOMO/LUMO gap usually decreases and, for large enough fields, energy levels can cross, which presages a change of electronic state and, if found in molecular electronic circuits, a change in transmission. The calculations also show changes in the geometry both of the molecule and the molecule/cluster interface upon application of the electric field. These effects should be anticipated in whole circuit studies.

The metastability of an electrochemically controlled nanoscale machine on gold surfaces.

The metastability of an electrochemically controlled nanoscale machine on gold surfaces.

Electron Binding to Nucleic Acid Bases. Experimental and Theoretical Studies. A Review

An in-depth knowledge of an excess electron binding mechanism to DNA and RNA nucleobases is important for our understanding of radiation damage influence on the biological functions of nucleic acids, as well as for the possible use of DNA molecules as wires in molecular electronic circuits. The of anions created by electron attachment to individual nucleic acid bases is discussed in detail. The principles of the experimental and theoretical approaches to the description of these anions are outlined, and the available results concerning valence- and dipole-bound anions of nucleic acid bases are reviewed. A review with 167 references.

Theoretical study of carotene as a molecular wire

Carotenoid molecules have important photo-chemical properties and may serve as molecular wires in a molecular electronic circuit. We have theoretically studied the intrinsic conducting properties of sulfur-terminated carotene between gold contacts using local orbital density functional theory. The dependence of the tunneling decay parameter “ β” on the degree of single–double bond alternation within the polyene backbone is determined from the polyene complex band-structure. The electron tunneling current–voltage characteristics is calculated using the Landauer–Büttiker formalism. The calculations are in qualitative agreement with experiments.

DNA as an engineering material

DNA is a wonderful material with which to build. It can act as a molecular glue or as the fuel for molecular engines. It can be made into rigid girders and tiles, and can also be used to design complex structures capable of self-assembly. DNA nanostructures can act as parallel computers, as “scaffolds” in protein-crystallography experiments and may soon form support structures in 3D molecular electronic circuits. The remarkable properties of DNA that allow it to be used as bricks, mortar and fuel could even lead to molecular assembly lines with conveyor belts that play an active role in the construction process.

Two-dimensional molecular electronics circuits.

Addressing an array of bistable [2]rotaxanes through a two-dimensional crossbar arrangement provides the device element of a current-driven molecular electronic circuit. The development of the [2]rotaxane switches through an iterative, evolutionary process is described. The arrangement reported here allows both memory and logic functions to use the same elements.

Self-assembled monolayer organic field-effect transistors.

The use of individual molecules as functional electronic devices was proposed in 1974 (ref. 1). Since then, advances in the field of nanotechnology have led to the fabrication of various molecule devices and devices based on monolayer arrays of molecules. Single molecule devices are expected to have interesting electronic properties, but devices based on an array of molecules are easier to fabricate and could potentially be more reliable. However, most of the previous work on array-based devices focused on two-terminal structures: demonstrating, for example, negative differential resistance, rectifiers, and re-configurable switching. It has also been proposed that diode switches containing only a few two-terminal molecules could be used to implement simple molecular electronic computer logic circuits. However, three-terminal devices, that is, transistors, could offer several advantages for logic operations compared to two-terminal switches, the most important of which is 'gain'-the ability to modulate the conductance. Here, we demonstrate gain for electronic transport perpendicular to a single molecular layer ( approximately 10-20 A) by using a third gate electrode. Our experiments with field-effect transistors based on self-assembled monolayers demonstrate conductance modulation of more than five orders of magnitude. In addition, inverter circuits have been prepared that show a gain as high as six. The fabrication of monolayer transistors and inverters might represent an important step towards molecular-scale electronics.

Photoinduced charge separation and recombination under distance, orientation, and spin controlled conditions

Great efforts have been made to mimic the efficient photoinduced charge separation and concomitant energy storage of natural photosynthetic systems via artificial (supra) molecular constructs as well as to design molecules with potential use for application in molecular electronic circuits. Close packing of such molecules introduces the problem of short‐circuiting and cross talk between the separate molecular components. In the present paper the limits will be investigated to which such short‐circuiting can be prevented by the use of saturated hydrocarbon (alkane) type insulating structures. Furthermore, as will be shown, several typically molecular scale operating principles such as orbital symmetry and spin multiplicity control can allow the realisation of systems in which fast and efficient charge separation is combined with very slow charge recombination even when the distance between the D(onor) and A(cceptor) moieties is very small.