Carbon nanotube-based nonvolatile random access memory for molecular computing

Molecular and nanoscale materials and devices in electronics

Over the past several years, there have been many significant advances toward the realization of electronic computers integrated on the molecular scale and a much greater understanding of the types of materials that will be useful in molecular devices and their properties. It was demonstrated that individual molecules could serve as incomprehensibly tiny switch and wire one million times smaller than those on conventional silicon microchip. This has resulted very recently in the assembly and demonstration of tiny computer logic circuits built from such molecular scale devices. The purpose of this review is to provide a general introduction to molecular and nanoscale materials and devices in electronics.

General, Room-Temperature Method for the Synthesis of Isolated as Well as Arrays of Single-Crystalline ABO4-Type Nanorods

Single-crystalline BaWO(4) and BaCrO(4) nanorods of reproducible shape and of varying sizes have been controllably prepared using a simple, room-temperature approach, based on the use of porous alumina template membranes. Aligned BaWO(4) and BaCrO(4) nanorod arrays can be obtained by dissolving the template. Our facile technique, which is analogous to biomineralization, offers a promising and generalized methodology to prepare other types of free-standing ABO(4) nanorods and their corresponding nanorod arrays. Extensive characterization of these samples has been performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED), Raman spectroscopy, FT-infrared spectroscopy (FT-IR), and X-ray diffraction (XRD).

Functionalization of carbon nanotubes using phenosafranin

Spectroscopic analysis and atomic force microscopy (AFM) phase imaging studies show self-assembly of phenosafranin (PSF) to multiwalled carbon nanotubes (MWNTs). The shift in absorption spectra is associated with charge transfer of valence electrons from PSF to electron accepting sites on the MWNTs. The Raman-active disorder modes are used to fingerprint PSF attachment to MWNTs via defect states. AFM phase imaging was used to obtain a molecular topographic visual confirmation of PSF attached to the MWNT.

Self-Assembly of Mesoscopic Metal-Polymer Amphiphiles

The assembly properties of two- and three-component rod-like building blocks consisting of gold and polymer block domains have been investigated. These structures behave like mesoscopic amphiphiles and form a series of single-layer superstructures consisting of bundles, tubes, and sheets depending upon the compositional periodicity. Unlike molecular systems, the template used to initially synthesize them plays a critical role in the assembly process by prealigning them in a manner that facilitates their assembly by optimizing the correct collisional orientation upon dissolution of the template. Tubular structures with tailorable diameters can be assembled in a predictable manner on the basis of an estimate of the hybrid rod packing parameters.

GeO[sub 2] fibers: Preparation, morphology and photoluminescence property

Nanomicron to submicron fibers of GeO(2) have been prepared using poly(vinyl acetate) and germanium dioxide sol by electrospinning followed by high temperature calcination. The morphology of the fibers have been studied by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. X-ray diffraction indicates that the fibers are single crystal with hexagonal alpha-phase quartz-like structure. At room temperature, the fibers show photoluminescence under excitation at 325 nm. The fibers may have potential applications in one-dimensional optoelectronic nanodevices.

Attachment of DNA to the Carbon Fiber Microelectrode via Gold Nanoparticles for Simultaneous Determination of Dopamine and Serotonin

A novel biochemical sensor was fabricated on a carbon fiber microelectrode, which consisted of an inner layer of electrodeposited gold nanoparticles, as a nano-array electrode, and an outer layer of electrodeposited calf thymus ds-DNA at +1.5 V vs. SCE. This modified electrode was characterized by X-ray photoelectron spectroscopy, scanning electron microscope, atomic force microscopy, cyclic voltammetry and differential pulse voltammetry (DPV). It was found that this electrochemical sensor exhibits a strong catalytic activity toward the oxidation of dopamine (DA), serotonin (5-HT) and ascorbic acid (AA), as a result of resolving the anodic voltammetric peaks of DA, 5-HT and AA into three well-defined peaks. Simutaneous DPV determination of DA and 5-HT can be achieved in the presence of 2000-fold AA. The modified electrode shows good sensitivity, selectivity and stability.

Phase Switching of a Single Isomeric Molecule and Associated Characteristic Rectification

Variations in molecular electronic structures related to conformational change are exceedingly attractive because of their key role in the understanding and development of functional processes in molecular electronics and biology. We observed, for the first time, the novel phase switching of a photoactive isomeric molecule, N-(2-mercaptoethyl)-4-phenylazobenzamide (Azo molecule) at a single-molecule level, which exhibits a distinctive change in the conductive characteristic under scanning tunneling microscope (STM) measurement. In comparison with the results obtained by the measurement of photoactive isomerization of the isolated Azo molecule, which was performed also for the first time, the observed characteristics are attributed to the results of the trans and cis phase transformation of the Azo molecule, under the condition of an external electric field and current flow. A specific point is that the potential landscape of the system is controllable by the electric field and provides a conformational stability with asymmetric bias dependence resulting in rectification.

Polymerization from the Surface of Single-Walled Carbon Nanotubes − Preparation and Characterization of Nanocomposites

Single-walled carbon nanotubes were functionalized along their sidewalls with phenol groups using the 1,3-dipolar cycloaddition reaction. These phenols could be further derivatized with 2-bromoisobutyryl bromide, resulting in the attachment of atom transfer radical polymerization initiators to the sidewalls of the nanotubes. These initiators were found to be active in the polymerization of methyl methacrylate and tert-butyl acrylate from the surface of the nanotubes. However, the polymerizations were not controlled, leading to the production of high molecular weight polymers with relatively large polydispersities. The resulting polymerized nanotubes were analyzed by IR, Raman spectroscopy, DSC, TEM, and AFM. The nanotubes functionalized with poly(methyl methacrylate) were found to be insoluble, while those functionalized with poly(tert-butyl acrylate) were soluble in a variety of organic solvents. The tert-butyl groups of these appended polymers could also be removed to produce nanotubes functionalized with poly(acrylic acid), resulting in structures that are soluble in aqueous solutions.

Nanowire Crossbar Arrays as Address Decoders for Integrated Nanosystems

The development of strategies for addressing arrays of nanoscale devices is central to the implementation of integrated nanosystems such as biological sensor arrays and nanocomputers. We report a general approach for addressing based on molecular-level modification of crossed semiconductor nanowire field-effect transistor (cNW-FET) arrays, where selective chemical modification of cross points in the arrays enables NW inputs to turn specific FET array elements on and off. The chemically modified cNW-FET arrays function as decoder circuits, exhibit gain, and allow multiplexing and demultiplexing of information. These results provide a step toward the realization of addressable integrated nanosystems in which signals are restored at the nanoscale.

Aligned Nanotubes

Since the discovery of carbon nanotubes by lijima in 1991, various carbon nanotubes with either a single- or multilayered graphene cylinder(s) have been produced, along with their noncarbon counterparts (for example, inorganic and polymer nanotubes). These nanostructured materials often possess size-dependent properties and show new phenomena related to the nanosize confinement of the charge carriers inside, which leads to the possibility of developing new materials with useful properties and advanced devices with desirable features for a wide range of applications. In particular, carbon nanotubes have been shown to exhibit superior properties attractive for various potential applications, ranging from their use as novel electron emitters in flat-panel displays to electrodes in electrochemical sensors. For many of the applications, it is highly desirable to have aligned/patterned forms of carbon nanotubes so that their structure/property can be easily assessed and so that they can be effectively incorporated into devices. In this Review, we present an overview on the development of aligned and micropatterned nanotubes, with an emphasis on carbon nanotubes.

Individual single-walled nanotubes and hydrogels made by oxidative exfoliation of carbon nanotube ropes

Single-walled carbon nanotubes were oxidized by a technique previously developed for the oxidation of graphite to graphite oxide (GO). This process involves treatment with concentrated H(2)SO(4) containing (NH(4))(2)S(2)O(8) and P(2)O(5), followed by H(2)SO(4) and KMnO(4). Oxidation results in complete exfoliation of nanotube ropes to yield individual oxidized tubes that are 40-500 nm long. The C:O:H atomic ratio of vacuum-dried oxidized nanotubes is approximately 2.7:1.0:1.2. XPS and IR spectra show evidence for surface O-H, C=O, and COOH groups. The oxidized nanotubes slowly form viscous hydrogels at unusually low concentration (>or=0.3 wt %), and this behavior is attributed to the formation of a hydrogen-bonded nanotube network. The oxidized tubes bind readily to amine-coated surfaces, on which they adsorb as smooth and dense monolayer films. Thin films of the oxidized nanotubes show ohmic current-voltage behavior, with resistivities in the range of 0.2-0.5 Omega-cm.

Nanoscale solid-state quantum computing

Most experts agree that it is too early to say how quantum computers will eventually be built, and several nanoscale solid-state schemes are being implemented in a range of materials. Nanofabricated quantum dots can be made in designer configurations, with established technology for controlling interactions and for reading out results. Epitaxial quantum dots can be grown in vertical arrays in semiconductors, and ultrafast optical techniques are available for controlling and measuring their excitations. Single-walled carbon nanotubes can be used for molecular self-assembly of endohedral fullerenes, which can embody quantum information in the electron spin. The challenges of individual addressing in such tiny structures could rapidly become intractable with increasing numbers of qubits, but these schemes are amenable to global addressing methods for computation.

Additional steps toward molecular scale wires: Further study of Ni(5)(10/11+) chains embraced by polypyridylamide ligands

This paper presents two advances in the development of the chemistry of extended metal atom chains (EMACs) that employ di(2-pyridyl)amide (dpa) and its higher homologues (loosely called polypyridylamides). As EMACs employing these ligands are extended to greater lengths, low solubility becomes an increasingly difficult problem. Also, increased stability would be desirable. We have employed a method, which is designed to be applicable to chains of any length, to introduce stabilizing substituents (ethyl groups) on some of the pyridyl rings. We illustrate this here by the synthesis and characterization of the pentanickel complexes Ni(5)(etpda)(4)Cl(2).6CHCl(3) and [Ni(5)(etpda)(4)](PF(6))(3).4Me(2)CO, etpda = the anion of N,N'-bis(4-ethylpyridyl)-2,6-diaminopyridine. As we had previously predicted, on the basis of the behavior of Ni(3)(dpa)(4)Cl(2) and [Ni(3)(dpa)(4)](PF(6))(3), oxidation causes marked changes in structure and magnetic behavior indicative of a change of electronic structure that would cause an insulator-conductor transformation. We now demonstrate that this is what occurs not only in the previously known Ni(5) compounds but in the new ethyl-substituted ones.

Growth of millimeter-long and horizontally aligned single-walled carbon nanotubes on flat substrates

Millimeter-long and well-aligned single-walled carbon nanotubes (SWNTs) have been produced on silica/silicon surfaces using the carbon monoxide chemical vapor deposition (CO-CVD) method. The orientation of the nanotube arrays can be well-controlled by the gas flow during the growth. The majority of the orientated SWNTs are straight and individual. The length of the nanotubes can be >2 mm for a 10 min growth. Furthermore, multidimensional crossed-networks of SWNT can be easily generated by multistep processes. These results present a great opportunity in the controllable production of organized SWNT arrays for large-scale carbon nanotube-based nanodevice fabrication.

Solubilization of single-wall carbon nanotubes by supramolecular encapsulation of helical amylose

We have developed a simple, efficient process for solubilization of single-wall carbon nanotubes (SWNTs) with amylose in aqueous DMSO. This process requires two important conditions, presonication of SWNTs and subsequent amylose treatment in an optimum mixture of DMSO/H2O. The former step separates SWNT bundles, and the latter step provides a maximum cooperative interaction of SWNTs with amylose, leading to the immediate and complete solubilization. The best solvent condition for this is around 10-20% DMSO, in which amylose assumes a random conformation or an interrupted helix. This indicates that the amylose helix is not the prerequisite for encapsulation of SWNTs. The SEM and AFM images of the encapsulated SWNTs manifest loosely twisted ribbons wrapping around SWNTs, which are locally intertwined as a multiple twist, but no clumps of the host amylose are seen on SWNT capsules.

The emerging field of nanotube biotechnology

Nanoparticles are being developed for a host of biomedical and biotechnological applications, including drug delivery, enzyme immobilization and DNA transfection. Spherical nanoparticles are typically used for such applications, which reflects the fact that spheres are easier to make than other shapes. Micro- and nanotubes--structures that resemble tiny drinking straws--are alternatives that might offer advantages over spherical nanoparticles for some applications. This article discusses four approaches for making micro- and nanotubes, and reviews the current status of efforts to develop biomedical and biotechnological applications of these tubular structures.

Carbon nanotubes from organometallic precursors

Multiwalled as well as single-walled carbon nanotubes are conveniently prepared by the pyrolysis of organometallic precursors such as metallocenes and phthalocyanines in a reducing atmosphere. More importantly, pyrolysis of organometallics alone or in mixture with hydrocarbons yields aligned nanotube bundles with useful field emission and hydrogen storage properties. By pyrolysis of organometallics in the presence of thiophene, Y-junction nanotubes are obtained in large quantities. The Y-junction tubes have a good potential in nanoelectronics. Carbon nanotubes prepared from organometallics are useful to prepare nanowires and nanotubes of other materials such as BN, GaN, SiC, and Si(3)N(4).

Carbon nanotubes--the route toward applications

Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products. Others are demonstrated in early to advanced devices, and one, hydrogen storage, is clouded by controversy. Nanotube cost, polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications of single-walled nanotubes.

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.

Distributed response analysis of conductive behavior in single molecules

The ab initio computational approach of distributed response analysis is used to quantify how electrons move across conjugated molecules in an electric field, in analogy to conduction. The method promises to be valuable for characterizing the conductive behavior of single molecules in electronic devices.

Phase coherent electronics: a molecular switch based on quantum interference

The phenomenon of quantum mechanical interference may be used to control the conductivity of ballistic molecular wires. Using a simple model we demonstrate plausible effects and discuss its potential uses for constructing coherence-based molecular electronics.

Formation of oriented molecular nanowires on mica surface

Molecular "nanowire" structures composed of the charge transfer complex of a bis-tetrathiafulvalene substituted macrocycle and tetrafluorotetracyanoquinodimethane were constructed on mica substrates by employing the Langmuir-Blodgett technique. The nanowires transferred from a dilute aqueous potassium chloride subphase had typical dimensions of 2.5 nm x 50 nm x 1 microm. The nanowires are oriented to specific directions, corresponding to the directions of the potassium-ion array on the mica surface having sixfold symmetry. Such correlation between the nanowires and the substrate surface was also observed when a dilute aqueous rubidium chloride subphase was used. On the other hand, the correlation completely disappeared when the subphase contained divalent cations, indicating that the molecular nanowires orient by recognizing the monocation array on the mica surface. The nanowires formed by the vertical dipping method coexist with the monolayers. Only nanowire structures are, however, observed when we apply the horizontal lifting method. Based on the crystal structure of a related complex, a possible structure of the nanowires is presented. The conductivity of the nanowires was estimated to be of the order of 10(-3) S x cm(-1). The nanowires formed specific (regular) structures such as T-shape junctions, suggesting their use in construction of future molecular nanoscale devices.

Inorganic semiconductor nanowires: rational growth, assembly, and novel properties

Rationally controlled growth of inorganic semiconductor nanowires is important for their applications in nanoscale electronics and photonics. In this article, we discuss the rational growth, physical properties, and integration of nanowires based on the results from the authors' laboratory. The composition, diameter, growth position, and orientation of the nanowires are controlled based on the vapor-solid-liquid (VLS) crystal growth mechanism. The thermal stability and optical properties of these semiconductor nanowires are investigated. Particularly, ZnO nanowires with well-defined end surfaces can function as room-temperature ultraviolet nanolasers. In addition, a novel microfluidic-assisted nanowire integration (MANI) process was developed for the hierarchical assembly of nanowire building blocks into functional devices and systems.