Molecular electronics describes a field that seeks to understand charge-transport properties of single molecules and their ensembles. Ultimately, this may pave the way for electronic components made of molecular building blocks. This visionary concept was first sketched out in 1974 by Arieh Aviram and Mark Ratner, who suggested that a single molecule could function as a rectifier in their theoretical work. If it comes true, computational architectures may be built atom by atom, or molecule by molecule, namely, in a “bottom-up” way. In the past 40 years, molecular electronics has generated more and more excitement supported by constantly growing theoretical and experimental achievements. On the one hand, the prospect of size reduction in electronics offers a potential alternative/addition to complementary metal–oxide–semiconductor (CMOS) technology using “top-down” methods, which is expected to face fundamental limitations on further scaling (e.g. direct source–drain electron tunneling) in the next decade or so. On the other hand, synthetic method promises a molecular-level control of properties. Well-designed organic molecules represent a class of inexpensive, flexible, and easy-processing materials compared with their inorganic counterparts. These advantages of organic materials have been well exploited in “bulk” thin-film devices, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic photovoltaics (OPVs), some of which have already been successfully commercialized. On this basis, in spite of even more challenges, many efforts have been concentrated on pushing organic electronics into the nano/atomic realms. To date, individual molecules equipped with unique optoelectronic properties have shown their potential for molecular-scale electronics.

However, silicon-based technology is unlikely to be replaced anytime soon, and there are some major issues along the way to molecular electronics. Before we can actually set out to build computers out of molecules, understanding charge transport of single molecules or a small collection of molecules is of fundamental importance. This requires making reliable contacts to molecular components, which means a degree of control with atomic accuracy. Development of scanning probe microscopy (SPM) in the 1980s enabled some of the earliest attempts to image and measure individual molecules on surfaces. By using a scanning tunneling microscopy (STM) or conducting atomic force microscopy (cpAFM) tip, single molecules can be repeatedly interrogated, and in this way fundamental insights into how electron transport occurs within molecules and their contacts can be provided. SPM techniques are still among the most important testbeds today and continuously making contributions to our understanding of the principles in the field of molecular electronics. However, from an application perspective, integrated circuits of metal–molecule–metal junctions require robust electrode geometries that can be prepared over large areas. Given the size of a molecule, typically a few nanometers, making electrical contacts with a nanometer gap (dimension varies depending on the actual size of a target molecule) is the first crucial step for the realization of practical molecular electronic devices. To this end, a variety of platforms have been experimentally developed to wire molecules into circuits with atomically defined precision, which are generally referred to as “nanogap electrodes.”

Fabrication of nanogap electrodes is a very challenging mission because it goes beyond the capability of traditional microfabrication technologies. Although the field of molecular electronics has been around for more than 40 years, it was not until late 1990s that researchers managed to measure single-molecule transport in a predefined nanogap. From then on, a collection of effective and creative methods for manufacturing nanogap electrodes with controlled spacing have been reported and some fundamental problems got overcome. Nowadays, researchers around the world are taking further steps toward more robust devices, deeper insight into molecule–electrode interfaces, multiterminal architectures, new materials for electrodes (e.g. carbon materials), and synthetic control on molecular functionalities. More significantly, both the experimental and theoretical advances have opened up a set of exciting possibilities of understanding and manipulating single-molecule properties beyond simple descriptions of electronic transport, such as quantum interference, thermoelectronics, mechanics, and spintronics.

This book focuses on the fundamental issues of nanogap electrodes and their applications in molecular electronics. It covers the topics of (i) introduction of molecular electronics and charge-transport mechanism in molecular junctions; (ii) techniques for the fabrication of nanogap electrodes and characteristics of various types of nanogap electrodes; (iii) characterization methods and analytical techniques for molecular transport junctions; (iv) experimental advances in electrical measurements of single molecules and self-assembled monolayers; (v) measurements in molecular junctions beyond simple electronic transport. The microfabrication methods for the preparation of nanogap electrodes and electrical measurements on the single-molecule level will be emphasized in this book. The state of the art of nanogap electrodes and their prospects (e.g. organic–inorganic hybrid structures, integration and application over large areas) will also be discussed.

We hope that this book will attract the attention of graduate students and young scientists who are interested in molecular electronics, as well as those more senior academic and industrial researchers related to nanoscience and technology. We also hope that this book will, in a more experimental perspective, improve people's understanding of this relatively new while immensely vivid field.

Finally, we would like to express our gratitude to Prof. Wenping Hu at Tianjin University for his full support on the book project, the contributors of each chapter for their great work, and editors Shirly Samuel, Daniela Bez, and Umar Saleem at John Wiley & Sons, Ltd for their patience and help through the editorial process.