Table of contents

Recently, biochemical systems have been shown to possess interesting computational properties. In a parallel development, the chemical computation metaphor is becoming more and more frequently used as part of the emergent computation paradigm in computer science. We review in this contribution the idea behind the chemical computational metaphor and outline its relevance for nanotechnology. We set up a simulated reaction system of mathematical objects and examine its dynamics by computer experiments. Typical problems of computer science, such as sorting, parity checking or prime number computation are placed within this context. The implications of this approach for nanotechnology, parallel computers based on molecular devices and DNA-RNA-protein information processing are discussed.

Mixtures of the polyaniline (emeraldine base) and phosphorylated calix[4]resorcinolarene 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 and 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 or HCl films are prepared by choosing the component ratio. Detection of and HCl in the ppm range is demonstrated.

Scanning tunneling microscopy (STM) has been used to observe naturally occurring nanostructures formed on the (110) and (100) faces of single-crystal Cu samples. On the (110) surface, a combination of chemical and thermal treatments result in the formation of a periodic array of Cu - O stripes separated by areas of the clean surface. Deposition of submonolayer coverages of N onto both the (100) and (110) faces of Cu results in nanometer-scale islands. On the (100) surface, periodic arrays of N islands are formed. The feasibility of using these islands to create new nanostructures is demonstrated. On the (110) surface, the ability to combine O- and N-induced nanostructures to form an `atomic-scale tartan' is demonstrated.

Helical logic is a theoretical proposal for a future computing technology using the presence or absence of individual electrons (or holes) to encode 1s and 0s. 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 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 .

Friction force microscopy is implemented in ultrahigh vacuum conditions. Atomic-scale friction is observed on clean surfaces. The onset of atomic-scale stick - slip is observed and discussed in relation to the plucking model. Comparative measurements of AgBr(001) versus NaCl(001) and versus NaCl(001) demonstrate the material-specific contrast of friction. Contrast mechanisms are discussed in relation to these experiments. The role of the chemical nature of the tip is exemplified for the case of Si(111) , where wearless sliding is achieved with a PTFE-coated probing tip.

This special issue presents papers based on talks given at the Engineering Foundation Conference on Ordered Molecular and Nanoscale Electronics held in Keauhou-Kona, Hawaii, USA in June 1994. The meeting was chaired by Professor Mark Reed of Yale University, with co-chairs Doctors Avi Aviram and Phaedon Avouris of IBM T J Watson Research Center, and was supported by the Office of Naval Research and the Advanced Research Projects Agency. The papers in this issue have been edited with the assistance of Diane Maggs of the Pennsylvania State University, to whom we are most grateful. As seen in the diversity of these papers, most of which are updated from the original talks given, the topics of the meeting and this issue include: novel devices and fabrications methods, molecular conductors, quantum dots, single electron structures, self-assembly, and atomic manipulation.

P S Weiss Department of Chemistry The Pennsylvania State University University Park, PA16802-6300 USA

M A Reed Department of Electrical Engineering Yale University New Haven, CT 06520-8284 USA

We have studied the nanomechanical properties of thin films epitaxially grown on GeS(001) substrates by scanning force methods. The local frictional coefficient derived for islands was found to be significantly smaller than on the GeS(001) substrate demonstrating that well ordered films can lower the frictional force even compared with a layered material. In the second part of our study, we have used a scanning force microscope (SFM) for nanomechanical modification of a variety of thin film substrates including high- superconductors and thin metallic films on insulating substrates. A combination of photolithography and SFM-based nanofabrication allowed to link the nanoscopic to the macroscopic world and to perform transport measurements on the nanofabricated structures.

Carbon enhanced vapor etching (CEVE) is an alternative approach for fine pattern delineation in silicon dioxide, especially for nanolithographic processes. Exposures by a scanning electron microscope have achieved etch rates of about with etch selectivity ratios of 30 or greater for exposed to unexposed areas. Scanning tunneling microscope resolution studies have shown that trench widths on the order of 3 - 5 nm are possible. Pattern transfer to silicon has been achieved using reactive ion etching.

Chemical reactivity on Si(111)- at the atomic scale as it occurs spontaneously, or as it is induced by phenomena occurring between the tip and the Si substrate in a scanning tunneling microscope (STM), is investigated in the presence of as a reactant gas. The STM has been modified for this purpose to operate in the corrosive environment and in temperatures in the range of 250 - 300 K.

Reactant molecules are adsorbed on the surface at a low temperature which prevents desorption, diffusion, or spontaneous reactions during the time span necessary for STM-induced experiments. A voltage pulse is applied to the STM tip located above the adsorbed molecule to generate a highly localized and strong electric field. The adsorbed molecule undergoes a chemical reaction to form volatile fluoride resulting in removal of a silicon atom from the surface. The volatile product species is identified as . The Si atoms neighboring the reaction site undergo minimum displacement as a result of the process.

The spontaneous reaction kinetics of on the Si surface at low temperatures, after low coverage deposition, is also studied. Low temperatures provide the necessary time to acquire STM information on the number of adsorbed molecules before any substantial reaction takes place. In time, the adsorbed molecules either desorb or react with Si surface atoms to produce volatile products and therefore leave vacancies behind. Once the surface is free of adsorbed fluorinated molecules, excess vacancies resulting from the reaction are counted and the true removal probability is calculated.

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.

We investigated the low temperature transport properties of AlGaAs/GaAs quantum dot arrays. The coupling between dots and the electron density are controlled by a single gate covering the array. Below 1 K, the current - voltage (I - V) curves show multiple discontinuous jumps in the current, or `switching events', between different insulating and conducting states, which occur at gate-voltage and temperature dependent thresholds. Each single switching event is accompanied by hysteresis, and multiple switching events result in a hierarchy of hysteresis loops. A possible mechanism for this behavior, involving gate-to-dot tunneling, is discussed.

Nanometer-sized particles manufactured by two different techniques and deposited on wet chemically treated Si(111) substrates are explored at room temperature with a scanning tunneling microscope (STM) in the light of their stability upon repeated imaging and the feasibility to manipulate them with the probing tip. The two preparation techniques include field-induced transfer of Au from the STM tip stimulated by voltage pulsing and a wet chemical method. In the latter case, a colloidal solution of specified admixtures is prepared delivering CdS particles with a diameter of approximately 5 nm. In STM images, they protrude as 50-nm-high isolated hillocks or as one particle-high islands containing a varying number of individual constituents. The STM imaging process does not cause any changes in the structure of the islands or in the position of single particles. Upon traversing the tip through particle islands, several constituents can be removed. Depending on the deposition conditions in the field-induced transfer mode, the created Au deposits either adhere strongly to the substrate or they are displaced by the STM tip while imaging.

Tunneling through quantum dots is determined by the interplay between charging effects and the discrete energy-level spectrum originating from the three-dimensional confinement. We have performed spectroscopic measurements of many-particle ground and excited states in a single quantum dot by studying the linear and nonlinear transport. The occupation of excited states can lead to the appearance of negative differential conductance and to a suppression of transport via the ground states of the system. In a double-quantum-dot system consisting of two quantum dots of different sizes the measured conductance through the system is influenced by the charging energies of the individual dots and the coupling between the two dots.

Although scanning probe microscopy is traditionally limited to slow temporal response, techniques utilizing nonlinear tip-to-sample interactions can be used to capture very fast temporal signals (voltages, in our case). Such SPM-based techniques may deliver unrivaled spatial and temporal resolution. We have developed a scanning force microscope probe for measuring ultrafast voltage signals with picosecond time resolution. Measurements of VLSI and MMIC devices are shown.

A novel three-terminal switching device, fabricated only from metals, has recently been demonstrated. Somewhat similar to a semiconductor transistor, the physical principles of operation are quite different. It is an active device driven by a thermodynamic force associated with the effective Zeeman energy of the spin polarized electrons in the base. `Bipolar' has a double meaning: there are two polarities of carriers, upspin and downspin electrons; and the output can be a positive or negative voltage (or current). The three-terminal device does not have power gain, but because it shows a memory effect it is natural to use it as a storage element in a nonvolatile memory array. Power gain can be achieved in a five-terminal embodiment, allowing fanout and the linking of devices in logic operations. Switching times faster than 1 ns can be expected. Because all the layers are metals and charge carrier densities are high, fabrication of the device at submicron scales is quite possible and operation at the nanometer scale is conceivable. Thus, the possibility of achieving high packing densities is very plausible, and is not obviated by problems common to semiconductors such as heat dissipation and low carrier density.

We discuss the use of a conducting-tip atomic force microscope (AFM) for the imaging and electrical measurement of chemically derived nanostructures. First, scanning probe microscopy of CdSe and Au nanocrystals bound to a substrate with a self assembled monolayer will be discussed. It is found that imaging in liquids is necessary to avoid removing the nanocrystals. We then address some issues in performing electrical measurements in liquids. In particular, we examine the conducting properties of the AFM tip when imaging a flat surface, highly oriented pyrolytic graphite, in a non-polar liquid, hexadecane. We find that the solvation layers between the tip and the substrate strongly influence the electrical properties.

We present a review of recent advances in the molecular rectifier project. Using a donor - -bridge - acceptor compound, specifically designed to be a molecular rectifier, drastically improved rectifier characteristics were observed compared with a previously published -bridged compound. In particular the increased `rectification ratio' of the -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, -bridged compound, - , 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.

We present results on microwave-assisted transport through quantum dots. First, the important energy/frequency scales are discussed. Then, measurements of the current versus gate voltage characteristics in the presence of microwaves are presented. At finite source-drain bias, microwave-induced features are observed, and at zero source-drain bias, an oscillating photocurrent is observed. A model of photon-assisted transport is discussed that can account for the experimental observations.

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.

We report on a synthetic strategy for fabrication of close-packed planar arrays of nanometer diameter metal clusters. The clusters are single fcc 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.

In a molecular electronic device, a key parameter of interest is the electronic coupling linking the various functional units (e.g. organic molecules/fragments, inorganic complexes, nano-electrodes). In practical applications, rate constants for specific electron-transfer processes can easily be evaluated considering all interactions within the system; often, exponential decreases in rate constants with increasing separation between functional units (bridge length) is found, and there is now considerable interest in finding systems with much slower, non-exponential decreases. The simple Hamiltonian model, first introduced in 1961 by McConnell, is qualitatively descriptive of most electron transfer processes and, with the aid of numerous analytical approximations, predicts an exponential bridge-length dependence. However, the range of validity of the approximations used is small and exponential falloff is known to be much more general and robust. We investigate the analytical solution recently obtained by Evenson and Karplus for various aspects of the problem and find that the most serious approximations used in analysing the McConnell Hamiltonian modify the value of the exponent rather than introduce non-exponentiality. Hence, we introduce some simple improved rate laws appropriate to both the exponential and non-exponential regimes. Also, the analysis is extended to consider important systems bridged by and/or bonds, in which the bridge band structure is more complex: similar rate laws are found to apply, and indeed all results obtained are expected to be generally descriptive.

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 Å, sufficient for trans-membrane spanning; this length may be e.g. doubled in one synthetic step, producing an octamer of length about 120 Å, sufficient for electrode spanning.

The synthesis of oligo(2-ethylphenylene-ethynylene)s, oligo(2-(-ethylheptyl)phenylene-ethynylene)s, and oligo(3-ethylthiophene-ethynylene)s is described via an iterative divergent convergent approach. Synthesized were the monomer, dimer, tetramer, octamer and 16-mer of the oligo(3-ethylthiophene-ethynylene)s and oligo(2--ethylheptyl)phenylene-ethynylene)s. The 16-mers are 100 Å and 128 Å long, respectively. At each stage in the iteration, the length of the framework doubles. Only three sets of reaction conditions are needed for the entire iterative synthetic sequence; an iodination, a protodesilylation, and a Pd/Cu-catalyzed cross coupling. The oligomers were characterized spectroscopically and by mass spectrometry. The optical properties are presented which show the stage of optical absorbance saturation. The size exclusion chromatography values for the number average weights, relative to polystyrene, illustrate the tremendous differences in the hydrodynamic volume of these rigid rod oligomers versus the random coils of polystyrene. These differences become quite apparent at the octamer stage. The preparation of thiol-protected end groups is described. These may serve as molecular alligator clips for adhesion to gold surfaces. These oligomers may act as molecular wires in molecular electronic devices and they also serve as useful models for understanding related bulk polymers.

AFM-generated surface modifications are used to fabricate free-standing Si nanostructures. We employ the local electric field of a metal-coated AFM tip which is operated in air to selectively oxidize regions of a H-passivated Si surface. The resulting oxide, thick, is used as a mask for deep selective etches of the unoxidized regions of Si. The etched structures reside on a buried oxide layer which is removed to produce free-standing Si wires and cantilevers. Due to the uniformity of the exposure and self-limiting etch processes, these structures are extremely uniform, which is a critical feature for nanometer-scale device applications.

Mixed composition monolayers of similar n-alkanethiols on 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.

We have imaged stable islands of benzene molecules on the surface of at 77 K using a low temperature ultrahigh vacuum scanning tunneling microscope. The islands consist of a small number of benzene molecules and nucleate in straight lines on the surface. The ordering and stability of these clusters are discussed in terms of the molecular interactions with the substrate surface state electrons and intermolecular interactions.

Microcontact printing techniques employing self-assembled alkanethiol monolayers in the production of metal masks have been combined with 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.

This paper describes applications in microfabrication using patterned self-assembled monolayers (SAMs) formed by microcontact printing. Microcontact printing is a flexible new technique that forms patterned SAMs with regions terminated by different chemical functionalities (and thus different physical and chemical properties), in patterns with 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 ) as nm resists. Patterned SAMs formed by can also control the wettability of a surface on the scale. The organization of liquids in patterned arrays with dimensions, and the patterned deposition of microcrystals and microcrystal arrays illustrate the use of controlled wettability for microfabrication.