Advances in Nanoengineering: Electronics, Materials and Assembly

The following sections are included:

• PREFACE

• CONTENTS

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Carbon is one of the elements most abundant in nature. It is essential for living organisms, and as an element occurs with several morphologies. Nowadays, carbon is encountered widely in our daily lives in its various forms and compounds, such as graphite, diamond, hydrocarbons, fibers, soot, oil, complex molecules, etc. However, in the last decade, carbon science and technology has enlarged its scope following the discovery of fullerenes (carbon nanocages) and the identification of carbon nanotubes (rolled graphene sheets). These novel nanostructures possess physico-chemical properties different to those of bulk graphite and diamond. It is expected that numerous technological applications will arise using such fascinating structures. This account summarizes the most relevant achievements regarding the production, properties and applications of nanoscale carbon structures. It is believed that nanocarbons will be crucial for the development of emerging technologies in the following years.

High aspect ratio inorganic structures with at least one dimension between 1 and 100 nm, such as nanotubes, nanowires, and nanorods, have attracted a keen interest both for fundamental research and for advanced applications because of their unique and fascinating properties, largely contributing to the rise of nanoscience as a new discipline of scientific progress worldwide. Although the synthesis of high aspect ratio nanostructures has been achieved through many different strategies, the exact mechanisms that lead to their formation are not clear and need to be elucidated to control the properties and reliability of these nanoscopic “building blocks”. Here I give few examples of growth methods based on chemical and physical processes, and I describe the results of the first characterization of the nanostructures by high-resolution electron microscopy techniques. The analysis of surfaces and interfaces, epitaxial relationships, and defects is expected to advance the understanding of the growth strategies and of the properties at the nanoscale. Far from being a complete review, this report is meant to show the achievements and the difficulties of experimental research in nanoscience, and to introduce the challenges that lay ahead.

Developments in the understanding of how materials behave enable us to design material structures to display specified properties. We introduce multilayered materials as systems in which new properties can emerge, not found in their constituents in bulk form. The importance of transmission electron microscopy to determine structure–property relationships in nanoscale multilayers through characterization of their atomic structure is emphasized. Two examples of technologically useful multilayer systems are considered in more detail: hard coatings made from nitride multilayer films and the new structures and magnetic properties that are found in some metal multilayer systems. Finally, we discuss the future developments that are required to fully exploit the novel properties found in multilayered materials.

With the benefit of millions of years of natural selection, it is no surprise that when it comes to fitting form to function, Nature is second to none in engineering the right structure for the right job. The creation of architectural elegance, despite using the minimal amount of material has long fascinated man, who has endeavored to understand how intricate construction can be accomplished merely through the flow of inorganic ions and strategically placed macromolecules. When the demands of the environment are the blueprints of the construction, structures are produced with the utmost efficiency. This was first stated explicitly by D'Arcy Thompson, in his seminal work On Growth and Form (1917).¹ However, even as far back as the 16th century, scientist and astronomer Johannes Kepler noted that “Nature uses as little as possible of anything”. Both these and many other luminaries held Nature in the highest esteem as an engineer par excellence.

In the 19th century, people used these tenets and took their inspiration for daring and innovative architectural and engineering projects from Nature. In the 21st century therefore, when advanced nanoscale materials are required, it is only right that man should once again turn to Nature for solutions to their creation.

Supramolecular chemistry is the chemistry of “designed noncovalent intermolecular interactions”. In this chapter we will trace the beginnings of supramolecular chemistry from molecular recognition through the more recent advances in the design of prototypical molecular machines.

Among the many challenges facing the development of a molecular-based nanotechnology, the directed assembly of discrete molecular objects, and their controlled integration into macroscopic structures, is fundamental. The selective self-assembly or self-organizing characteristic inherent to certain molecules, for example DNA, is a property that could be exploited to address these challenges. This integration problem can be separated into more fundamental tasks: attaching molecular anchors to the macroscopic structures with high spatial resolution; assembling the discrete molecular objects; and positioning and attaching the molecular objects onto the macroscopic structures.

Here, several aspects of these tasks will be discussed, for example using short DNA molecules as molecular anchors and their attachment to electrodes separated by a few ten nanometers; the generation of branched DNA complexes by molecular self-organization; and using AC electric fields for the manipulation, orientation, and positioning of DNA molecules.

Significant advances in molecular electronics require an ability to reliably analyze the relationship between molecular and electronic characteristics. The spatial resolution of conductive proximal probes has much to offer in this regard. Previous work has shown that protein molecules can be controllably assembled on planar electrode surfaces and subsequently trapped between the conductive surfaces presented by a metal-coated AFM probe and an underlying planar substrate. This chapter reviews our recent efforts in analyzing the conductance of these metalloprotein junctions and includes discussions of relevant work in the area.

This chapter discusses two limitations which restrict the design of modern electronic circuits. The first is how small a circuit can be made, which is ultimately governed by the size of the individual electrons inside a circuit. Physicists are probing this limit, and in doing so are finding ways to control with great precision the movement of single electrons. The second limitation is the operational frequency of electronic systems, which is currently restricted to frequencies below approximately 1 THz. At frequencies in the terahertz range, the concepts governing electronic circuits design start to merge with concepts from the field of photonics. Experiments which are probing these two limitations are discussed in the context of their implications for future microelectronic circuit design.

A quantum device is an electronic component whose electrical properties can only be understood from quantum mechanical phenomena, such as the wave nature of electrons or the discreet energy levels of a confined electron. A couple of criteria must be met before quantum observations can be made. First, the thermal energy must be less than the electron energy-level spacing which typically requires a combination of temperatures less than 1 K (−272°C) and device dimensions less than one micron (micron = 1 μm = 10⁻⁶ m). Second, electron motion must be ballistic, meaning no scattering takes place within the confines of the device, which requires small devices and material of extremely high purity. Quantum devices have been studied intensely for the past 20 years, partly because they provide an ideal arena to investigate diverse quantum-mechanical phenomena, but also to identify and study the next generation of electronic components whose mass production will soon be possible if microchip miniaturization continues at the current rate. The simplest quantum device is a two-dimensional electron gas (2DEG), an electron sheet, where electrons are confined in one dimension and free in two. Confinement by electric fields occurs at the junction between layers of the compound semiconductors GaAs and AlGaAs. Confinement in further dimensions, to create quantum wires and dots for example, is usually achieved using patterned surface electrodes fabricated by electron beam lithography (EBL). During experiments a negative bias is applied to the surface electrodes which expel electrons from the 2DEG lying underneath the electrodes. The quantum wires or dots are created in the 2DEG plane under gaps between the electrodes. Erasable electrostatic lithography (EEL) is a flexible alternative to EBL. Using a charged scanning probe, patterns of negative charge are drawn on a GaAs sample surface. Electrons are expelled from the 2DEG underneath the surface charge, and the quantum device is created in the 2DEG plane under gaps in the surface charge. EEL has the unique advantages that fabrication takes place in the same low-temperature high-vacuum environment required for measurement and that patterning is erasable. The charge, and therefore the quantum device, can be erased either locally using the scanning probe or globally by illuminating the device with a flash of red light. This means that as part of the experiment, the device geometry can be modified without constraint and the device tuned for optimum performance. With EEL many new fascinating experiments become possible and often with reduced fabrication timescales.

Magnetic materials provide the most important form of erasable data storage for information technology today. The demand for increased storage capacity has caused the size of the region used to represent a “1” or “0” of binary data and features of the read–write transducers to be reduced to the nanometer scale. However, increased storage capacity is useful only if there is a commensurate reduction in the time taken to read and write the data. In this chapter the basic principles that determine the behavior of nanomagnetic materials are introduced and their use in data storage systems is described. Particular attention is paid to processes that limit the speed of operation of the data storage system. It is shown that optical pump–probe experiments may be used to characterize dynamic magnetic processes with femtosecond temporal resolution. The macroscopic magnetization of a ferromagnet can be made to precess in response to an optically triggered magnetic field pulse, leading to reduced switching times. Alternatively, an ultrashort laser pulse may be used to manipulate the magnitude of the magnetization on femtosecond time scales, leading to an ultrafast demagnetization in certain ferromagnets, and providing new insight into magnetotransport phenomena. Finally, the outlook for increased record and replay rates is assessed and the prospect of further use of optical techniques within magnetic data storage technology is discussed.

Optical imaging with nanoscale resolution, beyond that possible with conventional diffraction-limited microscopy, may be achieved by scanning a nano-antenna in close proximity to a sample surface. This review will first aim to provide an overview of the basic principles of the technique of scanning near-field optical microscopy, before moving on to consider its most widely implemented form, in which the sample is illuminated through a small aperture held less than 10nm to the sample surface, for optical imaging with a resolution of about 50 nm. The exciting new possibilities for high-resolution optical imaging and spectroscopy promised by “apertureless” near-field optical microscopy are then considered. Such techniques may involve local scattering of light from a sample surface by a tip, local enhancement of an optical signal by a metal tip, or the use of a fluorescent molecule or nanoparticle attached to a tip as a local optical probe of a surface. These new optical nanoprobes offer the promise of optical microscopy with true nanometer spatial resolution.

Single molecule detection has become instrumental in our understanding of the molecular world. The ability to perform experiments molecule by molecule and probe directly the static and dynamic disorder in a system has revealed new insights into a range of fundamental problems in physics, chemistry, and biology. These insights are now laying the foundations for engineering devices such as optical switches and photonic wires at the molecular level. Of the many single molecule techniques available, fluorescence microscopy has proved an invaluable tool by virtue of its high sensitivity, time-resolution, noninvasive nature, and compatibility with other spectroscopic techniques. The development of specific detection modalities has allowed a multitude of previously impracticable experiments to be realized, from photon antibunching to single particle tracking in living cells. Advances in fluorescent probe design, detectors, modes of acquisition and data analysis will undoubtedly push single molecule techniques to new limits of sensitivity and into new areas of application.

The following sections are included:

• INDEX