Electrospinning uses electrical forces to produce polymer fibres with nanometre-scale diameters. Electrospinning occurs when the electrical forces at the surface of a polymer solution or melt overcome the surface tension and cause an electrically charged jet to be ejected. When the jet dries or solidifies, an electrically charged fibre remains. This charged fibre can be directed or accelerated by electrical forces and then collected in sheets or other useful geometrical forms. More than 20 polymers, including polyethylene oxide, nylon, polyimide, DNA, polyaramid, and polyaniline, have been electrospun in our laboratory. Most were spun from solution, although spinning from the melt in vacuum and air was also demonstrated. Electrospinning from polymer melts in a vacuum is advantageous because higher fields and higher temperatures can be used than in air.
A direct, accurate and convenient procedure for calibrating the spring constants of probes used for force microscopy/spectroscopy is described. It amounts to deflecting an 'unknown' cantilever with a 'standard' lever, where the standard lever has been precalibrated. The absolute and relative accuracies of the procedure are +/-20-30% and +/-10-20%, respectively; the former is limited by uncertainties in the determination of the spring constant for the 'standard' lever, as well as that for the 'unknown'. The method differs from others of the static deflection variety by its exploitation of the routine features of current instruments. The technique is compared with other static and dynamic methods currently being used.
This paper describes applications in microfabrication using patterned self-assembled monolayers (SAMs) formed by microcontact printing. Microcontact printing (mu CP) is a flexible new technique that farms patterned SAMs with regions terminated by different chemical functionalities (and thus different physical and chemical properties), in patterns with mu m dimensions. Patterns of SAM are formed using an alkanethiol as an 'ink', and printing the alkanethiol on a metal support with elastomeric 'stamp'. We fabricate the stamp by moulding a silicone elastomer using a master prepared by optical or x-ray microlithography or by other techniques. SAMs of long-chain alkanethiolates on gold and other metals can act as nanometer resists by protecting the supporting metal from corrosion by appropriately formulated etchants: the fabrication of microstructures of gold and silicon demonstrates the utility of patterned SAMs (formed by mu CP) as nm resists. Patterned SAMs formed by mu CP can also control the wettability of a surface on the irm scale, The organization of liquids in patterned arrays with mu m dimensions, and the patterned deposition of microcrystals and microcrystal arrays illustrate the use of controlled wettability for microfabrication.
The dynamics of fluid flow through nanomachines is different from in other systems in that the flow is granular (no continuum assumption) and that the 'walls' move. We have performed molecular dynamics simulations of the flow of helium and argon inside carbon (graphite) nanotubes of several sizes. The fluid was started at some initial velocity; fluid particles were allowed to recycle axially through the tube via minimum image boundary conditions. Argon slowed down more quickly than helium. In addition, the behaviour of the fluid strongly depended on the rigidity of the tube; a dynamic tube slowed down the fluid far more quickly than one in which the tube was held frozen. It also depended on the fluid density and tube diameter. It did not, however, depend on the tube length, because fluid flow tended to prevent the development of strong longitudinal modes, whose behaviour is length dependent.
The current state of focused ion beam (FIB) applications in relation to solid state devices is reviewed, and recent use of FIB technology for lithography, etching, deposition, and doping are described. Etching and deposition have become essential processes for failure analysis and for mask repair in silicon ULSL production. Furthermore, the FIB doping technique has been used to fabricate quantum effect devices.
We describe two electrochemical self-assembly processes for producing highly ordered quasi-periodic arrays of quantum dots on a surface. The advantages of these techniques are: (i) they are 'gentle' and do not cause radiation damage to nanostructures unlike beam lithography, (ii) they have high throughput and are amenable to mass production unlike direct-write lithography, (iii) structures can be delineated on non-planar substrates, and (iv) the techniques are potentially orders of magnitude cheaper to implement than conventional nanosynthesis. Samples produced by these techniques have been characterized by microscopy, optical and transport measurements, Auger and x-ray. These measurements reveal intriguing properties of the nanostructures. In this paper, we describe our initial results and show the promise of such techniques for low-cost and high-yield nanosynthesis.
In this review, we show how noncovalent bonding interactions between pi-electron rich aromatic ring systems (e.g. hydroquinone) and the pi-electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene) can be used to self-assemble novel molecular architectures which are not only interesting to us, because of their fascinating topologies, but also because they have the potential to be developed into molecular structures with switchable properties on the nanometre scale. The high efficiency observed in the self-assembly of a catenane, and its dynamic properties in solution, represent the first step in the design and self-assembly of other molecular assemblies better suited for the study of molecular switching processes. Therefore, a series of rotaxanes, mechanically-interlocked molecular compounds, consisting of a linear pi-electron rich dumbbell-shaped component and the pi-electron deficient tetracationic cyclophane as the cyclic component, have been self-assembled and evaluated. All of the so-called molecular shuttles show translational isomerism and one of them, comprising benzidine and biphenol recognition sites as the non-degenerate pi-electron rich sites, shows molecular switching properties when it is perturbed by external stimuli, such as electrons and protons. The versatility of our approach to nanoscale molecular switches is proven by the description of a series of molecular assemblies and supramolecular arrays, consisting of pi-electron rich and pi-electron deficient components, which display molecular switching properties when they are influenced by external stimuli that are photochemical, electrochemical and/or chemical in nature. However, the molecular switching phenomena take place in the solution state. Therefore, finally we describe how simple molecular structures can be ordered on to a solid support at the macroscopic level using Langmuir-Blodgett techniques. This is a necessary condition which must be fulfilled if we wish to construct supramolecular structures with device-like properties at the macroscopic level.
Mixed composition monolayers of similar n-alkanethiols on Au(111) are formed by self-assembly. While the average surface composition of these films accurately reflects the composition of the deposition solution, scanning tunneling microscopy and secondary ion mass spectroscopy measurements show that the films phase separate on the nanometer scale. Scanning tunneling microscopy has been used to follow molecular motions within these films. We discuss our observations in terms of the formation and stability of the phase-segregated domains, and their potential importance in nanoscale applications.
Microcontact printing techniques employing self-assembled alkanethiol monolayers in the production of metal masks have been combined with CF4/O-2 reactive ion etch for subsequent pattern transfer to silicon. Silicon feature sizes of about 300 nm have been demonstrated. Some inadequacies in the self-assembled monolayers (SAMs)-formed metal masks have been characterized by electron microscopy. Particularly, nickel etch control and metal feature edge definition remain problems to be solved if the process is to be employed in submicron feature production. Nickel patterns produced in the process and used as masks without the gold overlayer were successful as masks in the reactive ion etching (RIE) process. They also appear to give a somewhat improved edge definition over processes in which the gold layer remains.
During six months from April to September 1994, the author visited 42 laboratories in eight different European countries involved with research in nanostructures. The scientific thrusts point the way to new methods and properties of matter that may provide useful information. The potential applications of research in this field are likely to be found in information technology, sensors, and new materials, replete with the implications for many industrial interests. It is a suitable topic for the recently introduced 'focused technical assessment' activities of the Office of Naval Research Europe where the coordination for this effort was focused. Overviews of multidisciplinary efforts in fields of physics, electrical engineering, materials, chemistry, and biology are presented.
We present a novel approach in an effort to perform electrical measurements at the level of a single molecule. A mechanical controllable break junction is utilized in combination with a molecular deposition technique in fluid to obtain the system: metal-molecule-metal. The I-V curve of this system shows pronounced features over a large voltage scale.
Results are reported from two molecular-dynamics simulations designed to yield insight into the engineering of nanometre-scale structures. The first is the initial stages of the indentation of a silicon substrate by an atomically-sharp diamond tip. Up to an indentation depth of approximately 0.6 nm the substrate responds elastically and the profile of the disturbed region of the substrate normal to the surface reflects the shape of the tip apex. The disturbed region in the plane of the surface, however, reflects the symmetry of the substrate rather than that of the tip. As indentation progresses the damage to the substrate becomes irreversible, and the profile of the damage normal to the substrate surface approximately matches that of the tip, while the in-plane profile appears roughly circular rather than displaying the symmetry of either the tip or substrate. The tip maintains its integrity throughout the simulation, which had a maximum indentation depth of 1.2 nm. The second study demonstrates patterning of a diamond substrate using a group of ethynyl radicals attached to a diamond tip. The tip is designed so that the terrace containing the radicals has an atomically-sharp protrusion that can protect the radicals during a tip crash. At contact between the tip and substrate the protrusion is elastically deformed, and five of six chemisorbed radicals abstract hydrogen atoms during the 1.25 ps the tip is in contact with the surface. Displacement of the tip an additional 2.5 Angstrom, however, results in permanent damage to the protrusion with little deformation of the substrate.
The difficulties faced by conventional silicon technology over the next ten years have been widely publicized, along with the possibility for a slow-down and eventual stagnation in the power of integrated circuits. Surmounting this problem will require new initiatives in lithography, materials processing, and device architecture which must be carefully coordinated in order to evolve a manufacturable nanoelectronics. Here we present one long-term strategy which incorporates a number of attractive features, based upon recent research results from several different fields. Our goal is not to propose an alternative roadmap, but to expand discussion of long-term possibilities in future silicon technology.
Mixtures of the polyaniline (emeraldine base) and phosphorylated calixresorcinolarene derivative (CA) are proposed to prepare LB films for conductometric gas sensors. They are quite stable at the air-water interface and give LB films of high quality. The average thickness of the mixed monolayers is found to be 1.6 nm. The as-deposited films are insulating. Doping with HCl increases the conductivity up to between 0.6 x 10(-3) and 0.5 S cm(-1) which depends on the component ratio. The films containing more than 20 wt% of CA are doped reversibly in part. Thus, the films which are highly sensitive to either NH3 or HCl films are prepared by choosing the component ratio. Detection of NH3 and HCl in the ppm range is demonstrated.
Porphyrin-derived materials may be linked together by fusion to rigid coplanar aromatic bridges such as substituted anthracenes to form linear or branched oligomers. Here, we consider linear oligomers of free-base porphin and meso-tetra-aryl derivatives, such as the tetramer synthesized by Crossley and Burn. A number of semi-empirical quantum-chemical methods have been used to determine the geometries and electronic structures of the ground and excited electronic states. The inter-ring coupling responsible for electron or hole conduction is discussed as a function of oligomer size and the predicted molecular-wire characteristics outlined. Comparison with properties of other linear molecular wires are summarized, and possibilities of introducing switching capacity are indicated. The edge-to-edge length of the tetramer is about 56 Angstrom, sufficient for trans-membrane spanning; this length may be e.g. doubled in one synthetic step, producing an octamer of length about 120 Angstrom, sufficient for electrode spanning.
Helical logic is a theoretical proposal for a future computing technology using the presence or absence of individual electrons (or holes) to encode is and Os. The electrons are constrained to move along helical paths, driven by a rotating electric field in which the entire circuit is immersed. The electric field remains roughly orthogonal to the major axis of the helix and confines each charge carrier to a fraction of a turn of a single helical loop, moving it like water in an Archimedean screw. Each loop could in principle hold an independent carrier, permitting high information density. One computationally universal logic operation involves two helices, one of which splits into two 'descendant' helices. At the point of divergence, differences in the electrostatic potential resulting from the presence or absence of a carrier in the adjacent helix controls the direction taken by a carrier in the splitting helix. The reverse of this sequence can be used to merge two initially distinct helical paths into a single outgoing helical path without forcing a dissipative transition. Because these operations are both logically and thermodynamically reversible, energy dissipation can be reduced to extremely low levels. This is the first proposal known to the authors that combines thermodynamic reversibility with the use of single charge carriers. It is important to note that this proposal permits a single electron to switch another single electron, and does not require that many electrons be used to switch one electron. The energy dissipated per logic operation can very likely be reduced to less than 10(-27) J at a temperature of 1 K and a speed of 10 GHz, though further analysis is required to confirm this. Irreversible operations, when required, can be easily implemented and should have a dissipation approaching the fundamental limit of In 2kT.
We report on a synthetic strategy for fabrication of close-packed planar arrays of nanometer diameter metal clusters. The clusters are single fee crystals of gold, each encapsulated by a monolayer of alkyl thiol molecules. They are electronically coupled by means of aryl dithiol molecules. This structure, which is of interest for developing nanoscale electronics, is created using molecular self-assembly methods. It should prove possible to tune the conductivity of such arrays from the metallic limit to the insulating limit by controlling the size of the gold clusters and the strength of the electronic coupling between them.
Large arrays of parallel metallic nanowires ranging from 20-120 nm in width are fabricated using a general and relatively simple technique. Holographic laser interference exposure of photoresist and anisotropic etching are used to pattern the surface of InP(001) substrates into V-shaped grooves of 200 nm period. Subsequently metal is evaporated at an angle onto the V-grooved substrates, naturally resulting in thousands of ultra-narrow metallic wires in parallel. Resistance measurements proof that as-prepared wires are electrically continuous.
An attempt is made to study the electronic contribution to the elastic constants in ultrathin films of ternary and quaternary alloys in the presence of an arbitrarily oriented magnetic field on the basis of a new electron dispersion law. It is found, taking Hg1-xCdxTe and In1-xGaxAsyP1-y lattice matched to InP as examples of ternary and quaternary alloys, that the said contribution increases with increasing surface electron concentration and the band non-parabolicity enhances its magnitude. They decrease with increasing film thickness and alloy composition in various oscillatory manners. Moreover, the numerical values of the contributions in ternary materials are greater than those in quaternary compounds. An experimental method is suggested for determining the electronic contribution to the elastic constants in degenerate compounds having arbitrary dispersion laws. In addition, the well known results for wide-band-gap ultrathin materials in the absence of a magnetic field have been obtained as special cases of our generalized formulations under certain limiting conditions.
We present a review of recent advances in the molecular rectifier project, Using a donor-sigma-bridge-acceptor compound, specifically designed to be a molecular rectifier, drastically improved rectifier characteristics were observed compared with a previously published pi-bridged compound, In particular the increased 'rectification ratio' of the sigma-bridged compound was attributed to more effective current blocking under reverse bias. Furthermore, we demonstrate that photodiode-like properties are possessed by films of the zwitterionic, pi-bridged compound, C16H33-gamma Q3CNQ, using a transparent electrode construction. Finally, we provide an explanation for the observation that all non-centrosymmetric Langmuir-Blodgett film samples exhibit an exponential current-density/voltage dependence.